How Television Works

Television is certainly one of the most influential forces of our time. Through the device called a television set or TV, you are able to receive news, sports, entertainment, information and commercials. The average American spends between two and five hours a day glued to "the tube"!

Have you ever wondered about the technology that makes television possible? How is it that dozens or hundreds of channels of full-motion video arrive at your house, in many cases for free? How does your television decode the signals to produce the picture? How will the new digital television signals change things? If you have ever wondered about your television (or, for that matter, about your computer monitor), then read on! In this article, we'll answer all of these questions and more.

TV Pixels and Your Brain
Let's start at the beginning with a quick note about your brain. There are two amazing things about your brain that make television possible. By understanding these two facts, you gain a good bit of insight into why televisions are designed the way they are.

The first principle is this: If you divide a still image into a collection of small colored dots, your brain will reassemble the dots into a meaningful image. This is no small feat, as any researcher who has tried to program a computer to understand images will tell you. The only way we can see that this is actually happening is to blow the dots up so big that our brains can no longer assemble them, like this:

Most people, sitting right up close to their computer screens, cannot tell what this is a picture of -- the dots are too big for your brain to handle. If you stand 10 to 15 feet away from your monitor, however, your brain will be able to assemble the dots in the image and you will clearly see that it is the baby's face. By standing at a distance, the dots become small enough for your brain to integrate them into a recognizable image.

Both televisions and computer screens (as well as newspaper and magazine photos) rely on this fusion-of-small-colored-dots capability in the human brain to chop pictures up into thousands of individual elements. On a TV or computer screen, the dots are called pixels. The resolution of your computer's screen might be 800x600 pixels, or maybe 1024x768 pixels.

TV Motion and Your Brain
The human brain's second amazing feature relating to television is this: If you divide a moving scene into a sequence of still pictures and show the still images in rapid succession, the brain will reassemble the still images into a single, moving scene. Take, for example, these four frames from the example video:


Each one of these images is slightly different from the next. If you look carefully at the baby's left foot (the foot that is visible), you will see that it is rising in these four frames. The toy also moves forward very slightly. By putting together 15 or more subtly different frames per second, the brain integrates them into a moving scene. Fifteen per second is about the minimum possible -- any fewer than that and it looks jerky.

When you download and watch the MPEG file offered at the beginning of this section, you see both of these processes at work simultaneously. Your brain is fusing the dots of each image together to form still images and then fusing the separate still images together into a moving scene. Without these two capabilities, TV as we know it would not be possible.

The Cathode Ray Tube





A few TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images. LCDs and plasma displaysare other common technologies. It is even possible to make a television screen out of thousands of ordinary 60-watt light bulbs! You may have seen something like this at an outdoor event like a football game. Let's start with the CRT, however.

The terms anode and cathode are used in electronics as synonyms for positive and negative terminals. For example, you could refer to the positive terminal of a battery as the anode and the negative terminal as the cathode.

In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light bulb). The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons that naturally pour off a heated cathode into the vacuum.

Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.

Inside a CRT



As you can see in the drawing, there's not a whole lot to a basic cathode ray tube.



There is a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. However, in this diagram you can see no way to "steer" the beam -- the beam will always land in a tiny dot right in the center of the screen.

That's why, if you look inside any TV set, you will find that the tube is wrapped in coils of wires. On the next page, you'll get a good view of steering coils.

TV Steering Coils

The steering coils are simply copper windings (see How Electromagnets Work for details on coils). These coils are able to create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils, you can position the electron beam at any point on the screen.

TV Phosphors


Electrical fields may cause a large number of ghost sightings. Learn about electrical fields and find out how electrical fields relate to ghosts.

A phosphor is any material that, when exposed to radiation, emits visible light. The radiation might be ultraviolet light or a beam of electrons. Any fluorescent color is really a phosphor -- fluorescent colors absorb invisible ultraviolet light and emit visible light at a characteristic color.

In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor, it makes the screen glow. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together.

There are thousands of different phosphors that have been formulated. They are characterized by their emission color and the length of time emission lasts after they are excited.

The Black-and-White TV Signal


In a black-and-white TV, the screen is coated with white phosphor and the electron beam "paints" an image onto the screen by moving the electron beam across the phosphor a line at a time. To "paint" the entire screen, electronic circuits inside the TV use the magnetic coils to move the electron beam in a "raster scan" pattern across and down the screen. The beam paints one line across the screen from left to right. It then quickly flies back to the left side, moves down slightly and paints another horizontal line, and so on down the screen.

In this figure, the blue lines represent lines that the electron beam is "painting" on the screen from left to right, while the red dashed lines represent the beam flying back to the left. When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen, as represented by the green line in the figure. When the beam is "painting," it is on, and when it is flying back, it is off so that it does not leave a trail on the screen. The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top.

As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, gray and white across the screen. Because the lines are spaced very closely together, your brain integrates them into a single image. A TV screen normally has about 480 lines visible from top to bottom. In the next section, you'll find out how the TV "paints" these lines on the screen.

Painting the TV Screen
Standard TVs use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second but only half of the lines are painted per frame. The beam paints every other line as it moves down the screen -- for example, every odd-numbered line. Then, the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times per second. Most computer monitors use progressive scanning because it significantly reduces flicker.

Because the electron beam is painting all 525 lines 30 times per second, it paints a total of 15,750 lines per second. (Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on.)

When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can accurately paint the picture that the TV station or VCR sends. The TV station or VCR therefore sends a well-known signal to the TV that contains three different parts:

Intensity information for the beam as it paints each line
Horizontal-retrace signals to tell the TV when to move the beam back at the end of each line
Vertical-retrace signals 60 times per second to move the beam from bottom-right to top-left
Composite Video Signal



A typical composite video signal
A signal that contains all three of these components -- intensity information, horizontal-retrace signals, and vertical-retrace signals -- is called a composite video signal. A composite-video input on a VCR is normally a yellow RCA jack. One line of a typical composite video signal looks something like the image on this page.

The horizontal-retrace signals are 5-microsecond (abbreviated as "us" in the figure) pulses at zero volts. Electronics inside the TV can detect these pulses and use them to trigger the beam's horizontal retrace. The actual signal for the line is a varying wave between 0.5 volts and 2.0 volts, with 0.5 volts representing black and 2 volts representing white. This signal drives the intensity circuit for the electron beam. In a black-and-white TV, this signal can consume about 3.5 megahertz (MHz) of bandwidth, while in a color set the limit is about 3.0 MHz.

A vertical-retrace pulse is similar to a horizontal-retrace pulse but is 400 to 500 microseconds long. The vertical-retrace pulse is serrated with horizontal-retrace pulses in order to keep the horizontal-retrace circuit in the TV synchronized.
Color TV Screen






A color TV screen differs from a black-and-white screen in three ways:

There are three electron beams that move simultaneously across the screen. They are named the red, green and blue beams.
The screen is not coated with a single sheet of phosphor as in a black-and-white TV. Instead, the screen is coated with red, green and blue phosphors arranged in dots or stripes. If you turn on your TV or computer monitor and look closely at the screen with a magnifying glass, you will be able to see the dots or stripes.
On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen.

When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.
Color TV Signal




A color TV signal starts off looking just like a black-and-white signal. An extra chrominance signal is added by superimposing a 3.579545 MHz sine wave onto the standard black-and-white signal. Right after the horizontal sync pulse, eight cycles of a 3.579545 MHz sine wave are added as a color burst.

Following these eight cycles, a phase shift in the chrominance signal indicates the color to display. The amplitude of the signal determines the saturation. Here is the relationship between color and phase:

Burst = 0 degrees
Yellow = 15 degrees
Red = 75 degrees
Magenta = 135 degrees
Blue = 195 degrees
Cyan = 255 degrees
Green = 315 degrees

A black-and-white TV filters out and ignores the chrominance signal. A color TV picks it out of the signal and decodes it, along with the normal intensity signal, to determine how to modulate the three color beams.
TV Broadcasts
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The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal.



Now you are familiar with a standard composite video signal. Note that we have not mentioned sound. If your VCR has a yellow composite-video jack, you've probably noticed that there are separate sound jacks right next to it. Sound and video are completely separate in an analog TV.

You are probably familiar with five different ways to get a signal into your TV set:

Broadcast programming received through an antenna
VCR or DVD player that connects to the antenna terminals
Cable TV arriving in a set-top box that connects to the antenna terminals
Large (6 to 12 feet) satellite-dish antenna arriving in a set-top box that connects to the antenna terminals
Small (1 to 2 feet) satellite-dish antenna arriving in a set-top box that connects to the antenna terminals

The first four signals use standard NTSC analog waveforms as described in the previous sections. As a starting point, let's look at how normal broadcast signals arrive at your house.

A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels:

54 to 88 MHz for channels 2 to 6
174 to 216 MHz for channels 7 through 13
470 to 890 MHz for UHF channels 14 through 83

The composite TV signal described in the previous sections can be broadcast to your house on any available channel. The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal.

To the left of the video carrier is the vestigial lower sideband (0.75 MHz), and to the right is the full upper sideband (4 MHz). The sound signal is centered on 5.75 MHz. As an example, a program transmitted on channel 2 has its video carrier at 55.25 MHz and its sound carrier at 59.75 MHz. The tuner in your TV, when tuned to channel 2, extracts the composite video signal and the sound signal from the radio waves that transmitted them to the antenna.

VCR and Cable Signals
VCRs are essentially their own little TV stations. Almost all VCRs have a switch on the back that allows you to select channel 3 or 4. The video tape contains a composite video signal and a separate sound signal. The VCR has a circuit inside that takes the video and sound signals off the tape and turns them into a signal that, to the TV, looks just like the broadcast signal for channel 3 or 4.

The cable in cable TV contains a large number of channels that are transmitted on the cable. Your cable provider could simply modulate the different cable-TV programs onto all of the normal frequencies and transmit that to your house via the cable; then, the tuner in your TV would accept the signal and you would not need a cable box. Unfortunately, that approach would make theft of cable services very easy, so the signals are encoded in funny ways. The set-top box is a decoder. You select the channel on it, it decodes the right signal and then does the same thing a VCR does to transmit the signal to the TV on channel 3 or 4.
Satellite TV Signals



Small-dish satellite system
PHOTO COURTESY DIRECTV

Large-dish satellite antennas pick off unencoded or encoded signals being beamed to Earth by satellites. First, you point the dish to a particular satellite, and then you select a particular channel it is transmitting. The set-top box receives the signal, decodes it if necessary and then sends it to channel 3 or 4.

Small-dish satellite systems are digital. The TV programs are encoded in MPEG-2format and transmitted to Earth. The set-top box does a lot of work to decode MPEG-2, then converts it to a standard analog TV signal and sends it to your TV on channel 3 or 4.

Digital TV


Sony Wega 42" XBR Plasma TV with built-in HDTV tuner
PHOTO COURTESY SONY ELECTRONICS

The latest buzz is digital TV, also known as DTV or HDTV (high-definition TV). DTV uses MPEG-2 encoding just like the satellite systems do, but digital TV allows a variety of new, larger screen formats.

The formats include:

480p - 640x480 pixels progressive
720p - 1280x720 pixels progressive
1080i - 1920x1080 pixels interlaced
1080p - 1920x1080 pixels progressive

A digital TV decodes the MPEG-2 signal and displays it just like a computer monitor does, giving it incredible resolution and stability. There is also a wide range of set-top boxes that can decode the digital signal and convert it to analog to display it on a normal TV.

TV Goes Digital



The term "digital TV" is used in many different ways right now, depending on whom you're talking to. There's also the term "HDTV," which is the most advanced form of digital TV in use in the United States. The reason it gets confusing is because digital TV in the United States combines three different ideas.

The first idea that is new to digital TV is the digital signal.

Analog TV started as a broadcast medium. TV stations set up antennas and broadcast radio signals to individual communities. You can attach an antenna to your TV and pick up channels 2 through 83 for free. What you receive, as described earlier, is a single, analog composite video signal and a separate sound signal.

Digital TV started as a free broadcast medium as well. For example, in San Jose, Calif., you can tune in to about a dozen different commercial digital TV stations if you have a digital TV receiver and an antenna. The FCC gave television broadcasters a new frequency to use for their digital broadcasts, so until the digital transition is complete, each broadcaster has an analog TV channel and a digital TV channel. The digital channel carries a 19.39-megabit-per-second stream of digital data that your digital TV receives and decodes.

Each broadcaster has one digital TV channel, but one channel can carry multiple sub-channels if the broadcaster chooses that option. Here's how it works:

On its digital channel, each broadcaster sends a 19.39-megabit-per-second (Mbps) stream of digital data. Broadcasters have the ability to use this stream in several different ways. For example:

A broadcaster can send a single program at 19.39 Mbps.
A broadcaster can divide the channel into several different streams (perhaps four streams of 4.85 Mbps each). These streams are called sub-channels, and this type of broadcasting is called multicasting. For example, if the digital TV channel is channel 53, then 53.1, 53.2 and 53.3 could be three sub-channels on that channel. Each sub-channel can carry a different program.

The reason that broadcasters can create sub-channels is because digital TV standards allow several different formats. Broadcasters can choose between three formats:

480i - The picture is 704x480 pixels, sent at 60 interlaced frames per second (30 complete frames per second).
480p - The picture is 704x480 pixels, sent at 60 complete frames per second.
720p - The picture is 1280x720 pixels, sent at 60 complete frames per second.
1080i - The picture is 1920x1080 pixels, sent at 60 interlaced frames per second (30 complete frames per second).
1080p - The picture is 1920x1080 pixels, sent at 60 complete frames per second.

(The "p" and "i" designations stand for "progressive" and "interlaced." In a progressive format, the full picture updates every 60th of a second. In an interlaced format, half of the picture updates every 60th of a second.)

The 480p and 480i formats are called the SD (standard definition) formats, and 480i is roughly equivalent to a normal analog TV picture. When analog TV shows are upconverted and broadcast on digital TV stations, they're broadcast in 480p or 480i.

The 720p, 1080i and 1080p formats are HD (high definition) formats. When you hear about "HDTV," this is what is being discussed -- a digital signal in the 720p, 1080i or 1080p format.

Finally, the HD formats of digital TV have a different aspect ratio than analog TVs. An analog TV has a 4:3 aspect ratio, meaning that the screen is 4 units wide and 3 units high. For example, a "25-inch diagonal" analog TV is 15 inches high and 20 inches wide. The HD format for digital TV has a 16:9 aspect ratio, as shown below:

The type of signal, format and aspect ratio have all changed in the process of converting from analog TV to digital TV in the United States.

Digital Compression
The idea of sending multiple programs within the 19.39-Mbps stream is unique to digital TV and is made possible by the digital compression system being used. To compress the image for transmission, broadcasters use MPEG-2 compression, and MPEG-2 allows you to pick both the screen size and bit rate when encoding the show. A broadcaster can choose a variety of bit rates within any of the three resolutions.

You see MPEG-2 all the time on the Web on Web sites that offer streaming video. For example, if you go to iFilm.com, you will find that you can view streaming video at 56 kilobits per second (Kbps), 200 Kbps or 500 Kbps. MPEG-2 allows a technician to pick any bit rate and resolution when encoding a file.

There are many variables that determine how the picture will look at a given bit rate. For example:

If a station wants to broadcast a sporting event (where there is lots of movement in the scene) at 1080i, the entire 19.39 megabits per second is needed to get a high-quality image.
On the other hand, a newscast showing a newscaster's head can use a much lower bit rate. A broadcaster might transmit the newscast at 480p resolution and a 3-Mbps bit rate, leaving 16.39 Mbps of space for other sub-channels.

It's very likely that broadcasters will send three or four sub-channels during the day and then switch to a single high-quality show that consumes the entire 19.39 Mbps at night. Some broadcasters are also experimenting with 1- or 2-Mbps data channels that send information and Web pages along with a show to provide additional information.

How Cell Phones Work

From Delhi to Mumbai to To Chennai, no matter where you go you'll see someone talking on his or her cell phone. These days, cell phones provide an incredible array of functions, and new ones are being added at a breakneck pace. Depending on the cell phone model, you can:

Store contact information
Make task or to-do lists
Keep track of appointments and set reminders
Use the built-in calculator for simple math
Send or receive e-mail
Get information (news, entertainment, stock quotes) from the Internet
Play games
Watch TV
Send text messages
Take photos and videos
Integrate other devices such as PDAs, MP3 players and GPS receivers

You might hear terms like 4G, LTE, GSM and CDMA thrown around and wonder what they refer to. At its most basic, a cell phone is a radio -- an extremely sophisticated radio, but a radio nonetheless.

Cell-phone Frequencies


In half-duplex radio, both transmitters use the same frequency. Only one party can talk at a time.

In the dark ages before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit 40 or 50 miles (about 70 kilometers). It also meant that not many people could use radio telephones -- there just were not enough channels.

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.

A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.


In full-duplex radio, the two transmitters use different frequencies, so both parties can talk at the same time.Cell phones are full-duplex.

Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once.
Channels - A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more.
Range - A walkie-talkie can transmit about 1 mile (1.6 kilometers) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 kilometers) using a 5-watt transmitter. Cell phones operate within cells, and they can switch cells as they move around. Cells give mobile phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.

In a typical analog cell phone system in the United States, the cell phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, like this:



Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in nonadjacent cells. The two purple cells can reuse the same frequencies.

Each cell has a base station that consists of a tower and a small building containing the radio equipment. We'll get into base stations later. First, let's examine the "cells" that make up a cellular system.

Cell-phone Channels
A single cell in an analog mobile phone system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

A cell phone carrier typically gets 832 radio frequencies to use in a city.
Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)

Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.

Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure on the previous page, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.
The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system and controls all of the base stations in the region.

Cell-phone Codes
All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider.

Let's say you have a cell phone, you turn it on and someone tries to call you. Here's what happens to the call:

When you first power up the phone, it listens for an SID (see sidebar) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.
When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.
Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of the phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.
The MTSO gets the call, and tries to find you. It looks in its database to see which cell you are in.
The MTSO picks a frequency pair that your phone will use in that cell to take the call.
The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend.
As you move toward the edge of your cell, your cell's base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This handoff switches your phone to the new cell.




As you travel, the signal is passed from cell to cell. Let's say you're on the phone and you move from one cell to another -- but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, it'll actually be handed off to the other service provider.If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds.

The less amazing thing is that you may be charged insane rates for your roaming call. On most phones, the word "roam" will come up on your phone's screen when you leave your provider's coverage area and enter another's. If not, you'd better study your coverage maps carefully -- more than one person has been unpleasantly surprised by the cost of roaming. Check your service contract carefully to find out whether you're paying when you roam. Most of the larger phone companies do not charge for roaming within the U.S., but some of the discount companies do.

Internationally is another story. The roaming rates can be very high, assuming you have a phone that can work in multiple countries. Different countries use different cellular access technologies. More on those technologies later. First, let's get some background on analog cell phone technology so we can understand how the industry has developed.

Analog Cell Phones


Old school: DynaTAC cell phone, 1983
PHOTO COURTESY MOTOROLA, INC.

In 1983, the analog cell phone standard called AMPS (Advanced Mobile Phone System) was approved by the U.S. Federal Communications Commission (FCC) and first used in Chicago. AMPS uses a range of frequencies between 824 megahertz (MHz) and 894 MHz for analog cell phones. In order to encourage competition and keep prices low, the U. S. government required the presence of two carriers in every market, known as A and B carriers. One of the carriers was normally the local-exchange carrier (LEC), a fancy way of saying the local phone company.

Carriers A and B are each assigned 832 frequencies: 790 for voice and 42 for data. A pair of frequencies (one to transmit and one to receive) is used to create one channel. The frequencies used in analog voice channels are typically 30 kilohertz (kHz) wide -- 30 kHz was chosen as the standard size because it gives you voice quality comparable to a wired telephone.

The transmit and receive frequencies of each voice channel are separated by 45 MHz to keep them from interfering with each other. Each carrier has 395 voice channels, as well as 21 data channels to use for housekeeping activities like registration and paging.

A version of AMPS known as Narrowband Advanced Mobile Phone Service (NAMPS) incorporates some digital technology to allow the system to carry about three times as many calls as the original version. Even though it uses digital technology, it is still considered analog. AMPS and NAMPS only operate in the 800-MHz band and don't offer many of the features common in digital cellular service, such as e-mail and Web browsing.

Along Comes Digital
The first digital cell phones were the second generation (2G) of cellular technology. Digital phones use the same radio technology as analog phones, but they use it differently. Analog systems don't fully use the signal between the phone and the cellular network -- analog signals can't be compressed and manipulated as easily as true digital signals. This is why cable companies switched to digital -- to fit more channels within a given bandwidth.

Digital phones convert your voice into binary information (1s and 0s) and then compress it (see How Analog-Digital Recording Works for details on the conversion process). This compression allows between three and 10 digital cell phone calls to occupy the space of a single analog call.

Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the two to send digital information between the cell tower and the phone. Clever modulation and encoding schemes are required to convert the analog information to digital, compress it and convert it back again while maintaining an acceptable level of voice quality. All of this means that digital cell phones have to contain a lot of processing power.

Inside a Digital Cell Phone
On a "complexity per cubic inch" scale, cell phones are some of the most intricate devices people use on a daily basis. Modern digital cell phones can process millions of calculations per second in order to compress and decompress the voice stream.

If you take a basic digital cell phone apart, you find that it contains just a few individual parts:

A circuit board containing the brains of the phone
An antenna
A liquid crystal display (LCD)
A keyboard (not unlike the one you find in a TV remote control)
A microphone
A speaker
A battery

Inside a digital cell phone, youll find a circuit board, battery, speaker and more. Look inside a digital cell phone with photos and explanations of each part.

The circuit board is the heart of the system. The analog-to-digital and digital-to-analog conversion chips translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog. You can learn more about A-to-D and D-to-A conversion and its importance to digital audio in How Compact Discs Work. The digital signal processor (DSP) is a highly customized processor designed to perform signal-manipulation calculations at high speed.

The microprocessor handles all of the housekeeping chores for the keyboard and display, deals with command and control signaling with the base station and also coordinates the rest of the functions on the board.

The ROM and flash memory chips provide storage for the phone's operating system and customizable features, such as the phone directory. The radio frequency (RF) and power section handles power management and recharging, and also deals with the hundreds of FM channels. Finally, the RF amplifiers handle signals traveling to and from the antenna.

The display has grown considerably in size as the number of features in cell phones has increased. Most current phones offer built-in phone directories, calculators, games, calendars, notes, Web browsers, and cameras, as well as countless other applications, or apps, to serve practically any need or want.

The SIM card on the circuit board

Some phones store certain information, such as the SID and MIN codes, in internal Flash memory, while others use external cards that are similar to SmartMedia cards.

Cell phones have such tiny speakers and microphones that it is incredible how well most of them reproduce sound. As you can see in the picture above, the speaker is about the size of a dime and the microphone is no larger than the watch battery beside it. Speaking of the watch battery, this is used by the cell phone's internal clock chip.

What is amazing is that all of that functionality -- which only 30 years ago would have filled an entire floor of an office building -- now fits into a package that sits comfortably in the palm of your hand!
Cell Phone Network Technologies: 2G



In FDMA, each phone uses a different frequency.

Cell phone networks fall into three categories: 2G, 3G and 4G. In 2G networks, there are three common technologies used for transmitting information:

Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)

Although these technologies sound very intimidating, you can get a good sense of how they work just by breaking down the title of each one.

The first word tells you what the access method is. The second word, "division," lets you know that it splits calls based on that access method.

FDMA puts each call on a separate frequency.
TDMA assigns each call a certain portion of time on a designated frequency.
CDMA gives a unique code to each call and spreads it over the available frequencies.

The last part of each name is "multiple access." This simply means that more than one user can use each cell.

FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

Narrow band means "channels" in the traditional sense. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.


TDMA splits a frequency into time slots.


GSM and CDMA


In CDMA, each phone's data has a unique code.

TDMA is also used as the access technology for Global System for Mobile Communications (GSM). However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. Think of GSM and IS-136 as two different operating systems that work on the same processor, like Windows and Linux both working on an Intel Pentium III. GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the 850-MHz and 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. It is used in digital cellular and PCS-based systems. GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular system introduced by Motorola and used by Nextel. AT&T and T-Mobile use GSM.

CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range. Verizon, Sprint and most other U.S. carriers use CDMA, which means no SIM card.

All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code is used to recover the signal. Because CDMA systems need to put an accurate time-stamp on each piece of a signal, it references the GPS system for this information. Between eight and 10 separate calls can be carried in the same channel space as one analog AMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800-MHz and 1900-MHz frequency bands.

Ideally, TDMA and CDMA are transparent to each other. In practice, high-power CDMA signals raise the noise floor for TDMA receivers, and high-power TDMA signals can cause overloading and jamming of CDMA receivers.

2G is a cell phone network protocol. Click here to learn about network protocols for smartphones.

Cell-phone Network Technologies: 3G


Queen of the selfie, Kim Kardashian, takes a picture with her smartphone of herself and friend La La Anthony at a New York Knicks game.
DIGITAL VISION/THINKSTOCK

3G technology came along to support increased data needs. 3G stands for "third generation" -- this makes analog cellular technology generation one and digital/PCS generation two. 3G technology is intended for the true multimedia cell phone -- typically called smartphones -- and features increased bandwidth and transfer rates to accommodate Web-based applications and phone-based audio and video files.

3G comprises several cellular access technologies. Common ones include:

CDMA2000 - based on 2G Code Division Multiple Access
WCDMA (UMTS) - Wideband Code Division Multiple Access
TD-SCDMA - Time-division Synchronous Code-division Multiple Access

3G networks have potential transfer speeds of up to 3 Mbps (about 15 seconds to download a 3-minute MP3song). For comparison, the fastest 2G phones can achieve up to 144Kbps (about 8 minutes to download a 3-minute song). 3G's high data rates are ideal for downloading information from the Internet and sending and receiving large, multimedia files. 3G phones are like mini-laptops and can accommodate broadband applications like video conferencing, receiving streaming video from the Web, sending and receiving faxes and instantly downloading e-mail messages with attachments.

3G is a cell phone network protocol. Click here to learn about network protocols for smartphones.

Cell-phone Network Technologies: 4G


A woman displays a fourth-generation WiMAX device during a exhibition at a convention centrer in Taipei in 2010.
SAM YEH/AFP/GETTY IMAGES

The "4G" cell phone has been around for years – at least since 2006, if you go by Sprint's definition of 4G [source: Segan]. That original 4G network used a technology called WiMAX, one of several approaches to 4G implementation. Like previous generations, 4G is not a standardized designation. It's simply the next step up in speed.

All the top service providers have some type of 4G network now, in various stages of development and coverage, built on one of three main cellular-access technologies: LTE, HSPA+, and WiMAX [source: Gaylord].

WiMAX is somewhat unique among cell phone access methods in that it's based on 802.16 wireless standards – aka wireless broadband Internet which is used for homes and offices. The WiMAX approach takes the second iteration of those standards, 802.16e, which supports mobile access, and applies it to the cell phone realm [source: Phone Scoop]. Transmissions use orthogonal frequency division multiplexing, or OFDM, a method of dividing signal data into multiple channels to speed up delivery and then combining all the bits back into a single unit at the destination [source: 4G Americas]. WiMAX transmission can theoretically top out around 40 Mbps, but in reality it is much less [source: Segan].

HSPA+, on the other hand, is an upgrade to a long-existing approach to cellular: HSPA, or High-Speed Packet Access. HSPA builds on the 3G WCDMA infrastructure, which carries signals in either one or two frequency bands depending on mode [source: Tech FAQ]. In 3G form, HSPA networks have peak data-transfer speeds of 14.4 Mbps [source: 4G Americas]. By using higher-order QAM (quadrature amplitude modulation, which encodes multiple data streams into a single transmission, mimicking increased bandwidth), carriers can achieve HSPA+ speeds of up to 21 Mbps [sources: Tech Target, 4G Americas, Ruddock]. Antenna improvements may further increase HSPA+ transmission rates [source: 4G Americas].

4G LTE
For decades, the mobile world has pushed for industry-wide standards to sync up the technology and allow for definable advances. Many see LTE, or Long Term Evolution, as the first real chance at standardization, as many top carriers have signed on to adopt the technology [source: 4G Americas].

LTE is being developed as the 4G standard, which is why you sometimes see "4G LTE" as opposed to simply "4G." 4G could mean support for any speed above 3G; 4G LTE means support for up to 86 Mbps based on specific technology and software infrastructures [source: 4G Americas].

The LTE network is based on Internet Protocol (IP) standards, the kind that delivers Web pages to your computer, and adds voice data to the transmission streams [source: 4G Americas]. It uses a schematic called OFDMA, or Orthogonal Frequency Division Multiple Access, which is similar to the OFDM approach in WiMAX. OFDMA also separates the bits in a single data transmission into multiple subcarriers to increase speed, reassembling it at the destination. The LTE protocol, though, has the added ability to assign particular data paths to particular users on the fly, optimizing the bandwidth available at any given time [source: 4G Americas].

LTE can operate on a wide range of radio frequency bands, which will allow many mobile carriers to switch over to LTE without starting from scratch [source: 4G Americas]. The migration has already begun: 4G LTE is operating in many U.S. and European cities in 2013 [source: Osborne]. It doesn't require a new phone. LTE can operate alongside 2G and 3G networks, and multi-mode phones can access any of them, using LTE where it's available and, say, HSPA where it's not [source: 4G Americas].

Having a multi-mode phone, then, is a huge benefit as LTE towers start popping up around the country and around the world.

Multi-band vs. Multi-mode Cell Phones
If you travel a lot, you will probably want to look for phones that offer multiple bands, multiple modes or both. Let's take a look at each of these options:

Multiple band: A phone that has multiple-band capability can switch frequencies. For example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or a 1900-MHz system. A quad-band GSM phone could use GSM service in the 850-MHz, 900-MHz, 1800-MHz or 1900-MHz band.
Multiple mode: In cell phones, "mode" refers to the type of transmission technology used. So, a phone that supported AMPS and TDMA could switch back and forth as needed. It's important that one of the modes is AMPS -- this gives you analog service if you are in an area that doesn't have digital support.
Multiple band/Multiple mode: This best of both worlds allows you to switch between frequency bands and transmission modes as needed.

Changing bands or modes is done automatically by phones that support these options. Usually the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the digital mode(s) first, then switch to analog.

You can find both dual-mode and tri-mode phones. The term "tri-mode" can be deceptive. It may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog. In that case, it is a true tri-mode phone. But it can also mean that it supports one digital technology in two bands and also offers analog support. A popular version of the tri-mode type of phone for people who do a lot of international traveling has GSM service in the 900-MHz band for Europe and Asia and the 1900-MHz band for the United States, in addition to the analog service. Technically, this is a dual-mode phone, and one of those modes (GSM) supports two bands.

Of course, none of this would be possible without those soaring towers that carry cell phone signals from phone to phone.

Cell-phone Towers


Cell phone towers come in many shapes and sizes.

A cell phone tower is typically a steel pole or lattice structure that rises hundreds of feet into the air.

Pictured here is a tower with three different cell phone providers riding on the same structure. If you look at the base of a tower, you can see provider equipment.

The box houses the radio transmitters and receivers that let the tower communicate with the phones. The radios connect with the antennae on the tower through a set of thick cables.

If you look closely, you will see that the tower and all of the cables and equipment at the base of the tower are heavily grounded.

One sure sign that multiple providers share a tower is a five-way latch on the gate. Any one of five people can unlock this gate to get in.

How Guns & Bullets work

Bullets and missiles

Has any other single invention changed history quite so much as explosives? As the power behind bombs and missiles, chemical explosives have made possible most of the great wars of the last 1000 years or so, altering the course of history time and time again. Before the invention of gunpowder, the first chemical explosive, people had to fight their enemies hand-to-hand on the battlefield with crude weapons like swords and spears. Today, you don't even have to be able to see your enemy—let alone touch him: it's easy to drop bombs from airplanes, shoot them from submarines, or launch them on rockets from one side of the Earth to the other. But even though modern missiles are incredibly sophisticated, the basic science and technology behind them is pretty much the same as it was 1000 years ago!

Photo: Oil painting of a sailor loading ammunition, 1942, by McClelland Barclay (1891–1943), who was killed onboard ship by a Japanese torpedo during his service as a US war artist. Picture courtesy of the US Department of the Navy/Navy Historical Center.
How guns fire bullets

Bullets and missiles come in all shapes and sizes. At 21.8 meters (71 ft) long, one of the world's biggest intercontinental ballistic missiles, the US Air Force LGM-118A Peacekeeper, is three times the length of a station wagon (estate car)! But it works pretty much the same way as a handgun bullet the size of your pinkie.
What's inside a bullet?



Bullets are a bit like fireworks and they are arranged in three sections: the primer, the propellant, and the bullet proper. At the back, the primer (or percussion cap) is like the fuse of a firework: a small fire that starts a bigger one. The next section of the bullet, effectively its "main engine," is a chemical explosive called a propellant. Its job is to power the bullet through the air from the gun to the target. The front part of the bullet is a tapering metal cylinder that hits the target at high speed. It tapers to a point mainly to reduce air resistance, so it goes faster and further, but also to help it penetrate metal, flesh, or whatever else the target may be made from (it must penetrate the target before it can do damage).

Artwork: The three main parts of a bullet. 1) The primer "launches" the bullet by igniting the propellant. 2) The propellant accelerates the bullet down the gun. 3) And the bullet proper (the red and yellow bit at the end) is the part that actually does the damage. This one has a complete outer casing known as a full-metal jacket, which means it can be fired faster and further, but it retains its shape on impact. Bullets with a softer point spread out on impact and do more damage, but don't travel as fast or far.

What happens when you fire?

Bullets are designed to be (relatively) safe until the moment when you fire them. When you pull the trigger of a gun, a spring mechanism hammers a metal firing pin into the back end of the bullet, igniting the small explosive charge in the primer. The primer then ignites the propellant—the main explosive that occupies about two thirds of a typical bullet's volume. As the propellant chemicals burn, they generate lots of gas very quickly. The gas shoots from the back of the bullet, increasing the pressure behind it, and forcing it down the gun barrel at extremely high speed (300 m/s or 1000 ft/s is typical in a handgun).



The propellant chemicals in a handgun bullet are not designed to explode suddenly, all at once: that would blow the whole gun open and very likely kill the person firing it. Instead, they are supposed to start burning relatively slowly, so the bullet moves off smoothly down the gun. They burn faster as the bullet accelerates down the barrel, giving it a maximum "kicking" force just as it comes out of the end. As the bullet emerges, the whole gun recoils (leaps backward) because of a basic law of physics called "action and reaction" (or Newton's third law of motion). When the gas from the explosion shoots the bullet forwards with force, the whole gun jolts backwards with an equal force in the opposite direction.

Photo: Launch of a Peacekeeper missile. By Don Sutherland, courtesy of Defense Imagery.

The explosion that fires a bullet happens in the confined space of the gun barrel. As the bullet flies out of the gun, the pressure of the explosion is suddenly released. That's what makes a gun go BANG! It's a bit like uncorking a bottle of wine at much higher speed and pressure. Some bullets also make noise because they go so quickly. The fastest bullets travel at around 3000 km/h (over 1800 mph) —about three times the speed of sound. Like a supersonic (faster-than-sound) jet fighter, these bullets make shock waves as they roar through the air.
How bullets travel



Photo: Unlike a conventional weapon, this 75mm recoilless rifle doesn't jerk back when fired. It's open at the back so the explosive blast escapes from the rear of the gun, eliminating the usual recoil. You can clearly see the heat of the explosive charge exploding from the front and the blast simultaneously shooting out from the rear. The gunner barely moves at all. By Blake R. Waltman, courtesy of US Army.

Gun barrels have spiraling grooves cut into them that make bullets spin around very fast as they emerge. A spinning bullet is like a gyroscope: a sort of "stubborn" spinning wheel that always tries to keep turning the same way. If you try to tilt a gyroscope while it's spinning, it will try to resist whatever force you apply and, if you let go, it will soon tilt back the other way. This is why, when things are spinning, they are very hard to deflect from their path. We call this idea gyroscopic inertia or stability. A bullet behaves in exactly the same way: once it's spinning, it follows a straighter path as it goes through the air, so it's harder to deflect and much more likely to reach its target.

We think of bullets flying in perfectly straight lines—but nothing could be further from the truth. Several different forces act on a bullet as it goes through the air. Over very short distances, bullets do follow more or less a straight line. Over longer distances, they follow a slight downward curve because gravity tugs them toward the ground as they go along. Air resistance and the spinning, gyroscopic motion of a bullet complicate things too. Usually, because of recoil, the person firing wobbles the gun slightly when the bullet emerges. When all these factors—the bullet's motion, gravity, air resistance, recoil, and spinning—add together, they make a bullet follow a very complicated corkscrew path as it flies through the air.
Why bullets do damage





A moving object has momentum, which is the product of its mass and its velocity. The faster something moves and the heavier it is, the more momentum it has. A truck trundling along slowly has a lot of momentum because it weighs so much. Even though bullets are tiny, they have lots of momentum because they go so fast. And because they go fast, they also have huge amounts of kinetic energy, which they get from the chemical energy of the burning propellant. (Remember that kinetic energy is related to the square of an object's velocity—so if it goes twice as fast, it has four times the energy.)

Bullets do damage when they transfer their energy to the things they hit. The faster something loses its momentum, the more force it produces. (One way to define force is as the rate at which an object's momentum changes.) A rifle bullet coming to a stop in a tenth of a second produces as much force as a heavy, slow moving truck coming to rest in 10 seconds. Imagine being hit by a truck—and you'll have some idea why bullets do so much damage!

Photo: Left: Close-up of a bullet hole in the fuselage of a US Air Force plane, fired on while delivering relief supplies in Somalia. Picture by TSgt. Val Gempis courtesy of US Air Force and Defense Imagery.

Photo: Right: Now that's what I call kinetic energy! This is what happens when you fire a 7g (0.25 oz) projectile at a velocity of 25,000 km/h (16,000 mph) into a cast aluminum block. This huge hole has been made by something weighing about as much as an iron nail! Even if something is that tiny, if it's traveling at very high speed it will have enough kinetic energy to do a lot of damage. Photo by R.D. Ward courtesy of Defense Imagery.
More energy = more damage?

It's easy to conclude from this that a bullet needs to have as much energy as possible to do the maximum amount of damage but, unfortunately, it's not quite that simple. A rifle bullet has many times the velocity and kinetic energy of a handgun bullet, so much so that it will typically enter one side of a target, whiz straight through, and fly out the other side. If a bullet leaves the target at high speed, it's taking valuable energy with it. So what we really want from a bullet is that it deposits as much energy as possible inside the target, either stopping entirely without exiting or leaving with the minimum possible velocity. There are various ways to achieve this.

The crudest way is for a bullet to expand as it enters the target. A bullet that expands has a bigger cross-sectional area, so it creates a bigger hole (or wound) in the target. It takes more energy to make a bigger hole in something: we need to use more force over the same distance, so we say the the bullet "does more work" and uses more energy in the process. Bullets can be designed to expand by making them hollow at the pointed end and, after impact, they expand and squash down into a shape that looks like a button mushroom; that's why deforming bullets are called hollow-point or mushrooming bullets (Dum-Dum bullets is another common name for them, taken from the place in India where they were invented in the late 19th century). International law has restricted the use of expanding bullets like this in wartime since 1899, but some police forces do still use them. That's partly because expanding bullets do so much damage that they immediately incapacitate their target, but also because a mushrooming bullet, fired in self-defense (maybe in a crowded city street), is much more likely to stay inside its target and less likely to injure an innocent bystander by accident. Soft-point bullets work in a similar way, only using a soft lead tip instead of a hollow point, but expand more slowly and typically penetrate deeper.



Chart: Different types of bullets carry very different amounts of energy. With small bullets and relatively modest muzzle velocities, handguns are the least energetic weapons. At the opposite end of the spectrum, a Browning Machine Gun (BMG) .50 cartridge weighing about 50g (1.7oz) and traveling at about 900m/s (2000mph) carries almost 20,000 joules of energy—about 50–100 times more than a small handgun bullet. Chart data from various sources including "Ch3: Mechanisms of Injury/Penetrating Trauma" by J. Christopher DiGiacomo and James F. Reilly in The Trauma Manual by Andrew B. Peitzman (ed). Lippincott Williams & Wilkins, 2002.

How Cruise Ships Work

History of Cruise Ships
Everyone likes to go on vacation, right? Well, during the latter part of the 19th century, the world developed an appetite for travel. Trave­ling by sea was nothing ne­w, but it wasn't exactly speedy. Thomas Newcomen changed that in 1712 when he invented the steam engine, a revolutionary way to harness kinetic energy and convert it to power.

As the steam engine evolved, so did its uses, until in 1819 the first American ship aided by a steam engine crossed the Atlantic. The S.S. Savannah left from the U.S. city bearing its name on May 22, 1819, and arrived in Liverpool, England, 29 days later. While the Savannah only used its steam engine for approximately 85 hours (roughly 12 percent of the trip), the voyage made history, and the era of the steamship began [source: Columbia Encyclopedia].

The passenger ship industry flourished in the late 1800s and early 1900s, courtesy of the steam engine and of the growing number of American immigrants crowding ocean liners. Trans-Atlantic passage was still primarily one way, although affluent passengers traveled back and forth between England and New York for business or holiday.

Ambitious British shipping company White Star Lines began aggressively building the first fleet of ocean liners in 1849 and would revolutionize trans-Atlantic passage over the next 60 years. White Star set records in size and grandeur, building, among others, three large ships dubbed Olympic Class liners. The Olympic, Britannic and Titanic broke the mold of traditional ocean liners, and their speed and interior features made other ships look obsolete. But ocean travel was on the verge of changing.

The popularity of trans-Atlantic sea passage gradually declined with the arrival of the airplane. People could fly to more destinations in a fraction of the time it took on an ocean liner, so shipping companies changed their business model to focus on tourism instead of passenger transportation. In 1900, the American-Hamburg Company built the first ship specifically designed for cruises. The Prinzessin Victoria Luise measured 406 feet (124 meters) long by 52 feet (16 meters) wide, and was 4,409 gross tons [source: Norway-Heritage]. (In nautical terms a ton, typically called a gross register ton or gross ton, does not measure weight; it represents 100 cubic feet of internal capacity.)

During the early 1930s, Nazi leader Adolf Hitler boosted the fledgling cruise industry by offering holiday packages to German workers as part of a state-sponsored effort to unite the nation. Hitler eventually commissioned several new ships for service, making the Nazi Party early pioneers of the cruise ship industry.

How Cruise Ships Float

The massive U-shaped hull of the Cunard liner Queen Elizabeth as she undergoes her annual overhaul.

In order for ships to journey across the open sea, they must withstand the tremendous burden, or weight of the ship, along with the crew, luggage, supplies and passengers. They do that with a little help from the principles of density and buoyancy. Cruise ships can weigh upwards of 71,500 tons (65,000 tonnes). They displace the equivalent amount of water when they press down on the ocean, which meanwhile pushes up and keeps the ship afloat, or buoyant.

That's why when engineers talk about how heavy a ship is, you'll hear them talk about displacement instead of weight. To keep from sinking, the cruise ship has to displace its weight in water before it's submerged. That's a lot easier to do if the cruise ship is constructed in a way so that it's less dense than the water below it. Think of it as the difference between dropping a bowling ball in the water and trying to submerge a beach ball. The bowling ball can't displace enough water before it's submerged, so it sinks. The beach ball does the opposite and floats.






Engineers help ships to achieve buoyancy by choosing lightweight, sturdy materials and dispersing the weight of the ship across the hull. The hull, or body of the ship below the main deck, is typically very wide and has a deep base line, or bottom. Large ships such as freighters, naval vessels and transport and cruise ships commonly utilize displacement hulls, or hulls that push water out of the way, to stay afloat.

A round-bottom displacement hull looks like a large rectangle with rounded edges to dissipate drag, or the force exerted against a moving object. The rounded edges minimize the force of the water against the hull, allowing large, heavy ships to move smoothly along. If you somehow hoisted a cruise ship out of the water and looked at it standing a few hundred feet away, the hull would look like a huge capital letter "U," depending on the size of the keel. The keel runs from the bow to the stern and acts as the backbone of the ship.

Like just about everything in our lives, round-bottom displacement hulls have advantages and disadvantages. Unlike a boat with a v-hull, which rises out of the water and s­kirts the waves, round-bottom hulls move through the water, making them extremely stable and seaworthy. Passengers on these ships rarely feel any rocking or side-to-side movement.

Ships with round-bottom hulls move fluidly, but the resistance of the water makes them extremely slow. They can only go so fast before the addition of more engine power reaches a point of diminishing returns. Nevertheless, the need for stability and a smooth ride outweigh overall speed, thus making the round-bottom displacement hull a good fit for cruise ships.

The hull serves not only as stability but also as protection. Reefs, sandbars and icebergs can tear apart fiberglass, composite materials and even steel. To prevent catastrophic damage, shipbuilders typically construct cruise ships using extra-strength steel and insert double hulls as an extra precaution. A double-hull design is a hull within a hull, like a tire with an inner tube.

Unfortunately, accidents do happen. In order to prevent cruise ships from sinking should something penetrate the first two lines of defense, vertical watertight dividers known as bulkheads are installed throughout the interior of the hull. These dividers keep damaged ships afloat by containing incoming water into a compartment or compartments, thereby preventing the whole ship from flooding.

How Cruise Ships Move

The propellers on this superliner are easy to spot while this boat is dry-docked.

Without a source of power, these mammoth cruise ships would be nothing more than hotels drifting aimlessly. So what are the options? Many older cruise ships use reciprocating diesel engines to generate power for propulsion. The power of the engine is fed through a transmission to propeller shafts. Transmissions determine the revolutions of the propellers, much like the transmission transfers the engine RPM into manageable speeds to power the rear wheels in our cars. Modern cruise ships use either gas turbine or diesel electric engines as their power source for propulsion, as well as for the ship's systems. The larger the cruise ship, the greater the demand for electrical power. Some larger ships rely on two different power sources: one for propulsion and one exclusively for electrical power.

Aero derivative gas turbine engines generate heat that is converted from mechanical energy into electrical power. To achieve this, compressed air is ignited in a combustion chamber. The hot exhaust is forced over a turbine that spins to mechanically drive a shaft. This power can then be used to spin electrical generators. Diesel-electric engines work much the same way, yet use a direct drive system rather than a turbine. Output shafts are connected to electrical generators to produce electrical power.

Both engine types require fuel, and lots of it. For example, the QE2 consumes about 380 tons of fuel daily if it's traveling at a speed of 28.5 knots and carries enough fuel to sail nonstop for 12 days [source: Warwick]. Cruise ships usually fill up at various ports, using fueling barges like floating gas stations. They use a lower-grade diesel that tends not to burn as cleanly as road-going diesel-powered vehicles. No doubt when prices rise at the pump, cruise ships also feel the pinch.

All cruise ships rely on propellers to push them through water. Propellers, commonly referred to as screws, cut through the water and provide forward or reverse motion. Unlike airplanes, which require tremendous propeller speeds to provide the forward motion needed for flight, cruise ship propellers do not need to turn as fast. They rely on torque, or brute power, over RPM, or high speed. Therefore, cruise ships travel slowly, rarely topping 30 knots.

Newer, cutting-edge cruise ships, like the QM2, use azimuth thrusters, which are pods housing propellers that can rotate 360 degrees and provide optimum maneuverability. These thrusters replace rudders and are thought to have several benefits over conventional screw-type propeller systems, such as decreased stopping distance and greater fuel efficiency [source: AP]. They can be used with either gas turbine or diesel electric engines.

If the Titanic had been equipped with azimuth thrusters, things might have turned out differently. One of the Titanic's downfalls was its inability to turn quickly to avoid the iceberg that ripped the bow open. Traditionally, cruise ships rely on rudders and differentiating propeller speed to turn, but ships equipped with azimuth thrusters turn much more quickly, so more cruise ships are incorporating this technology.

Inside a Cruise Ship: Amenities and Jobs

Cruise ships are designed with serio­us comfort in mind, so they have a lot of amenitie­s and a lot of staff to oversee those amenities for the as many as 3,000 passengers. These thousands of passengers are spread out over the ship's multiple levels called decks. For instance, the QM2 has 13 separate decks and carries up to 3,056 passengers and a crew of 1,253. If you're on deck 13, you would be standing on the highest level of the ship, whereas if you ventured down to the first deck, you'd be closer to the bowels of the ship.

If you booked passage on the QM2, you would likely find yourself staying in one of the Britannia staterooms (or cabins) located in the middle decks. The smallest of these rooms (typically 155 square feet to 250 square feet) contain a king-size bed, closet, television, a phone and a desk. Go with money to blow, and you could find yourself rubbing elbows with the other high rollers on a separate deck where the deluxe apartments and suites are. One of these rooms can be as big as 2,250 square feet. That's as large as a four-bedroom house. Whatever you need -- whether it's an Xboxor a bottle of bubbly -- these are the accommodations where you'll get it fast.

After a good night's sleep, you would probably head to the upper decks to eat breakfast, choosing from one of the many restaurants. Rather than returning to your stateroom after breakfast, you might hang poolside, shoot some hoops, shop, check out the golf simulator or gamble. Of course, you could always venture off the ship if you're at a port of call.

After a packed day, nightfall brings with it more food. You may want to try a formal restaurant or grab something quick at one of the grills. With dinner done, the nightlife begins. If you're in the mood for the opera, you'll need to book a reservation at the Royal Court Theatre. Perhaps you just want to hang out, have a few drinks and dance at the Q32 Nightclub.

Wherever you are on the cruise ship and whatever you're doing, it's likely that a crew member isn't too far away since cruise ships employ very large crews, well over 1,000 people. Here's a list of some of the more nautically inclined crew members and some of their responsibilities.

Captain - Highest-ranking official on board. Responsibilities include navigation, crew management and executive decision-making. This is the person you'll want to dress up for if you're invited to dine at the captain's table.
Chief Officer - Responsible for training seamen and maintaining the ship
Chief Medical Officer - A medical doctor who can perform surgery and supervise all medical operations
Staff Captain - Also known as the executive officer and the captain's right-hand man
Chief Engineer - Oversees the mechanical aspects of the ship and its engines
Chief Radio Officer - Handles all communication, radar and weather systems

In addition, some­ of the staff focuses less on how the boat is running and more on how you're enjoying yourself. Since cruise ships truly are hotels on the sea, some of the folks you may encounter include bartenders, plumbers, electricians and ushers. Working on a cruise ship can be rewarding and financially appealing. Employees can earn up to five times what they might make on land, depending on which ship and company they work for.

How Motorcycles Work

The layout of the modern motorcycle was established by 1914 and has remained fundamentally unchanged ever since. The overall structure and function of a motorcycle is quite simple. It includes a gasoline engine, which converts the reciprocating motion of pistons into rotary motion, just like the engine in a car. A transmission system transmits this motion to the back wheel. As the back wheel turns, it propels the motorcycle forward. Steering is accomplished by turning the front wheel via the handlebars and by leaning the bike to one side or the other. Two hand levers enable the rider to operate the clutch and the front brake, while two foot pedals enable him to change gears and control the rear brake.

Motorcycle Engine


Inside a typical bike engine
Motorcycle engines work the same way that car engines do. They consist of pistons, a cylinder block and a head, which contains the valve train. The pistons move up and down in the cylinder block, driven by explosions of a fuel-air mixture that has been ignited by a spark. Valves open and close to allow the fuel-air mixture to enter the combustion chamber. As the pistons move up and down, they turn a crankshaft, which transforms the energy from the pistons into rotary motion. The rotational force of the crankshaft is transmitted, via the transmission, to the rear wheel of the motorcycle.

Motorcycle engines are generally classified by one of three characteristics: the number of cylinders they possess, the capacity of their combustion chambers or the number of strokes in their power cycles.

The Revolution, a Harley-Davidson V-twin engine

Cylinders

Motorcycle engines can have between one and six cylinders. For years, the V-twin design was the engine of choice for motorcycle engineers in America, Europe and Japan. The V-twin gets its name from the fact that the two cylinders form a V shape, such as the classic Harley-Davidson V-twin shown below. Notice the 45-degree angle in the Harley-Davidson V-twin -- other manufacturers may vary this angle to reduce vibration.

The V-twin is just one way to accommodate two cylinders. When the cylinders are oriented so that the pistons oppose each other, the result is an opposed-twin design. Parallel-twin engines have their pistons placed side by side in an upright position.

Today, the most popular design is the four-cylinder, which runs more smoothly and at higher revolutions per minute (rpms) than a comparable twin. The four cylinders can be placed in a row, or they can be arranged in a V-shape configuration, with two cylinders on each side of the V.


Capacity

The size of the combustion chamber in a motorcycle engine is directly related to its power output. The upper limit is about 1500 cubic centimeters (cc), while the lower limit is about 50 cc. The latter engines are usually found on small motorcycles (mopeds) that offer 100-miles-to-the-gallon fuel economy but only reach top speeds of 30 to 35 miles per hour.
Motorcycle Transmission

A simple transmission
A motorcycle engine can create an enormous amount of power, which must be delivered to the wheels of the vehicle in a controllable way. The motorcycle transmission delivers power to the rear wheel through a series of structures that include the gearset, the clutch and the drive system.

Gearset

A gearset is a set of gears that enable a rider to move from a complete stop to a cruising speed. Transmissions on motorcycles typically have four to six gears, although small bikes may have as few as two. The gears are engaged by shifting a lever, which moves shifting forks inside the transmission.


Clutch






The job of a clutch is to engage and disengage power from the engine crankshaft to the transmission. Without the clutch, the only way to stop the wheels from turning would be to turn off the engine -- an impractical solution in any kind of motorized vehicle. The clutch is a series of spring-loaded plates that, when pressed together, connect the transmission to the crankshaft. When a rider wants to shift gears, he uses the clutch to disconnect the transmission from the crankshaft. Once the new gear is selected, he uses the clutch to reestablish the connection.

The belt drive on a Buell Lightning

Drive Systems

There are three basic ways to transmit engine power to the rear wheel of a motorcycle: chain, belt or shaft. Chain final-drive systems are by far the most common. In this system, a sprocket mounted to the output shaft (i.e., the shaft in the transmission) is connected to a sprocket attached to the rear wheel of the motorcycle by a metal chain. When the transmission turns the smaller front sprocket, power is transmitted along the chain to the larger rear sprocket, which then turns the rear wheel. This type of system must be lubricated and adjusted, and the chain stretches and the sprockets wear, requiring periodic replacements.

The Friction Drive
The friction drive is another transmission found in some motorcycles. A friction drive is a type of continuously variable transmission, or CVT, in which the variation in gear ratios comes about as a disc connected to the engine (the driving disc) rotates across the face of a second disc connected to the rear wheel (the driven disc). By varying the radius of the contact point between the two disc surfaces, different gears can be achieved. Continuously variable transmissions have a long history of use in motorized vehicles, with variable friction transmissions appearing in motorcycles in the early 1900s.

Belt drives are an alternative to chain drives. Early motorcycles often used leather belts, which could be tensioned to give traction using a spring-loaded pulley and hand lever. Leather belts often slipped, especially in wet weather, so they were abandoned for other materials and designs. By the 1980s, advances in materials made belt final-drive systems viable again. Today's belts are made of cogged rubber and operate much the same way as metal chains. Unlike metal chains, they don't require lubrication or cleaning solvents.

Shaft final-drives are sometimes used. This system transmits power to the rear wheel via a drive shaft. Shaft drives are popular because they are convenient and don't require as much maintenance as chain-based systems. However, shaft drives are heavier and sometimes cause unwanted motion, called shaft jacking, in the rear of the motorcycle.

Motorcycle Chassis
The motorcycle chassis consists of the frame, suspension, wheels and brakes. Each of these components is described briefly below.

Frame

Motorcycles have a frame made of steel, aluminum or an alloy. The frame consists mostly of hollow tubes and serves as a skeleton on which components like the gearbox and engine are mounted. The frame also keeps the wheels in line to maintain the handling of the motorcycle.

This Harley-Davidson Softail has a swingarm rear suspension.

Suspension

The frame also serves as a support for the suspensionsystem, a collection of springs and shock absorbers that helps keep the wheels in contact with the road and cushions the rider from bumps and jolts. A swingarm design is the most common solution for the rear suspension. On one end, the swingarm holds the axle of the rear wheel. On the other end, it attaches to the frame via the swingarm pivot bolt. A shock absorber extends upward from the swingarm pivot bolt and attaches to the top of the frame, just beneath the seat. The front wheel and axle are mounted on a telescoping fork with internal shock absorbers and internal or external springs.

A tubeless front tire
for touring motorcycles


Wh­eels

Motorcycle wheels are generally aluminum or steel rims with spokes, although some models introduced since the 1970s offer cast wheels. Cast wheels allow the bikes to use tubeless tires, which, unlike traditional pneumatic tires, don't have an inner tube to hold the compressed air. Instead, the air is held between the rim and the tire, relying on a seal that forms between rim and tire to maintain the internal air pressure.

Tubeless tires are less likely to blow out than a tube-type tire, but on rough roads, they can be a problem because even a small bend in the rim can cause a deflation. Tires come in a variety of designs to match the needs of terrain and driving conditions. Dirt bike tires, for example, have deep, knobby treads for maximum grip on dirt or gravel. Touring bike tires, made of harder rubber, usually provide less grip but last longer. The tires of sportbikes and racers (generally steel-belted radials) deliver astonishing gripping power, especially considering the small area that is in contact with the road surface.

Parts of a disc brake

Brakes

The front and rear wheels on a motorcycle each have a brake. The rider activates the front brake with a hand lever on the right grip, the rear brake with the right foot pedal. Drum brakes were common until the 1970s, but most motorcycles today rely on the superior performance of disc brakes. Disc brakes consist of a steel braking disc, which is connected to the wheel and sandwiched between brake pads. When the rider operates one of the brakes, hydraulic pressure, acting through the brake line, causes the brake pads to squeeze against the disc on both sides. Friction causes the disc and the attached wheel to slow down or stop. Brake pads must be replaced periodically because the pad surfaces wear away after repeated use.

Seats and Accessories
Seats on motorcycles are designed to carry one or two passengers. They are located behind the gas tank and are easily removable from the frame. Some seats have small cargo compartments underneath or behind them. For more storage, saddlebags -- either hard plastic boxes or leather pouches -- can be installed on either side of the rear wheel or over the rear fender. Large motorcycles can even tow a small trailer or pull a sidecar. The sidecar has its own wheel for support and may have an enclosed seating compartment to accommodate a passenger.

Driving a Motorcycle




Many states require that motorcycle riders wear helmets. Helmets perform two functions in a crash. The outer shell, which is constructed of fiberglass or injection-molded plastic, distributes energy from an impact across a wider area. An inner lining made of polystyrene absorbs most of the shock of the impact.

Driving a motorcycle is very different from driving a car. Since motorcycles are two-wheeled vehicles, they topple over when ­they stop moving. A moving motorcycle is affected by gyroscopic forces that are unique to two-wheeled machines. As a result, new motorcyclists must develop the skills necessary to handle their machines and must be licensed before they can drive their motorcycles on the street. In particular, motorcycle riders must master the art of steering, braking and changing gears.

Steering

Steering a motorcycle at low speeds is a straightforward process. The rider simply turns the handlebar in the direction he wishes to go. This only works at speeds below five miles an hour. If a motorcycle is traveling any faster, the rider must use a different kind of steering, known as counter-steering. This type of steering may seem counterintuitive. That's because motorcycle riders must push the handlebars to the left to make the vehicle turn right and vice versa.

Here's how it would work out on the street. Imagine that you're riding a motorcycle on the interstate. In front of you, blocking the right half of your lane, is a wreck or some other obstacle. If you're a novice rider, you might be tempted to push on the right side of the handlebar, thinking this will turn the bike to left. In reality, this will steer the bike to the right, directly into the obstacle. Instead, you should push on the left side of the handlebar, which directs the front wheel to the right but steers the vehicle to the left.

Figure 1: The gyroscope is spinning on its axis.
Figure 2: A force is applied to try to rotate the spin axis.
Figure 3: The gyroscope is reacting to the input force along an axis perpendicular to the input force.


Why does a motorcycle work this way? The motorcycle's wheels act like gyroscopes and create gyroscopic forces when moving at speeds above five miles an hour. One of the most interesting effects related to a gyroscope is a phenomenon known as precession. When a force is applied perpendicular to a gyroscope's axis of rotation, the resulting motion is perpendicular to the input force. This motion is called precession, and it's what causes the steering in motorcycles to be counterintuitive. It's also why instructors often arm their beginning riders with a simple mnemonic: "Push left, turn left. Push right, turn right."

Braking

Stopping a moving motorcycle requires that the rider use two brakes -- one on the front wheel, controlled by the right hand, and one on the rear wheel, controlled by the right foot. Both brakes should be used at the same time, although the front brakes are more powerful and will typically provide 70 to 90 percent of the total braking force. New riders often fear using the front brake, but it should be applied every time a motorcycle is slowed or stopped. Many accidents are caused by riders braking incorrectly. According to the California Highway patrol, locking up the rear brakes is a factor in the majority of motorcycle crashes.


Changing Gears

Early motorcycle clutches were operated by a foot pedal in the same way that automobile drivers use clutches. This was awkward and dangerous because it required that the rider's left foot be off the ground when the bike came to complete stop (at an intersection, for instance). British designers solved this problem with a hand-operated clutch. Today, hand-operated clutches and foot-operated shifters are standard on all models.

Braking Advice

When a motorcycle experiences a rapid deceleration, weight shifts to the front wheel. This makes the back of the bike lighter and can result in the rear wheel locking up and skidding. In this situation, riders should simply keep the rear brake applied and focus their eyes on the horizon where they want the bike to go. The bike will continue to skid, but in a controllable manner with little fishtailing.

When the front wheel locks up, riders should ease off the front brake. If they don't, the front wheel can tuck under the bike, causing a fall. The best way to avoid a front lockup is to use a technique called "staged braking." In staged braking, the rider progresses through four stages, with each stage corresponding to a greater amount of pressure applied to the front brake:
Stage one has the rider applying the brake just to the point where there is the slightest friction between the brake pads and disc.
In stage-two braking, the rider progresses to stage one, then continues to apply a steadier force.
By stage four, which is usually reserved for emergencies that require rapid deceleration, the rider bears down on the brake as hard as possible, but only after progressing through the other stages.

This kind of progressive braking will serve motorcyclists in all driving situations and will usually prevent a front lockup.

Motorcycle History
Motorcycl­es evolved from the "safety" bicycle, a bicycle that offered many advantages in stability, braking and ease of mounting. The essential features of a safety bike included:
Spoked front and rear wheels of the same size - roughly 30 inches in diameter (compared to the "ordinary" bicycle, which had a 48-inch front wheel and a 30-inch rear wheel)
A chain-driven rear wheel
A front chainwheel roughly twice as large as the rear sprocket
A low center of gravity
Direct front steering

1901 Hendee Single

The first bicycle to provide all of these features and gain market acceptance was the Rover Safety, designed by John Kemp Starley in 1885. After the Rover pattern took over the market, safety bicycles were simply called "bicycles."

It didn't take long for someone to take the user-friendly safety bicycle design and strap on an internal combustion engine. The first to do so successfully was Gottleib Daimler, who is credited with building the first motorized bicycle -- or motorcycle -- in 1885. Daimler's motorcycle included a single-cylinder Otto-cycle engine mounted vertically in the center of the machine. It also had one wheel in front, one wheel in back and a spring-loaded outrigger wheel on each side for added stability. Its chassis consisted of a wooden frame and wheels with wood spokes and iron rims. Such designs were called "boneshakers" because of the rough, jarring ride they delivered.

The next notable motorcycle was designed in 1892 by Alex Millet. Millet incorporated the basic safety bicycle design, but added pneumatic tires and a five-cylinder rotary engine built into the rear wheel. The cylinders rotated with the wheel, while the crankshaft formed the rear axle.

The first Harley-Davidson was basically a motorized bicycle.

The Hildebrand & Wolfmueller was the first successful production two-wheeler, patented in Munich in 1894. More than 200 vehicles made it onto the road. Hildebrand & Wolfmueller decided to cool their parallel-twin engine with water, which required a water tank and radiator. Their solution was to build the coolant system into the top of the rear fender.

In 1895, DeDion-Buton introduced an engine that would revolutionize the motorcycle industry by making mass production possible. The DeDion-Buton engine was a small, light, high-revving four-stroke engine that could generate half a horsepower. Although DeDion-Buton used the engine in its motortricycles, motorcycle manufacturers around the world copied and used the design.

American production motorcycles were also based on the DeDion-Buton engine. The two most famous American motorcycle manufacturers to incorporate the DeDion-Buton engine, however, were the Indian Motorcycle Company and Harley-Davidson.

Motorcycles Go to War

A Harley-Davidson military motorcycle
When World War I started in 1914, the automobile did not own the roads. Motorcycles filled the gaps as dependable, reliable vehicles. In the war, their utilitarian nature was put to good use. American and European armies used motorcycles extensively to gather reconnaissance, deliver messages and, in some cases, engage in combat. In 1917, roughly one-third of all Harley-Davidson motorcycles produced were sold to the U.S. military; in 1918, that figure rose to 50 percent. By the end of the war, it is estimated that the Army used some 20,000 motorcycles -- most of them Harley-Davidsons.

Carl Oscar Hedstrom and George M. Hendee founded the Hendee Manufacturing Company in 1900 with the goal of producing a "motor-driven bicycle for the everyday use of the general public." In 1901, they rolled out the Single, a 1.75-horsepower motorcycle that could reach a top speed of 25 miles per hour. They also decided to roll out a brand-new trade name for their motorcycles. That name was Indian, and it was the world's best-selling motorcycle until World War I.

Founded by William S. Harley and Arthur Davidson in 1902, the Harley-Davidson Motor Company went on to produce the most influential machines of the industry. Its first models used the basic DeDion-Buton layout and borrowed heavily from chassis designs already employed by other motorcycle manufacturers, including Indian, Excelsior and Pope. The Harley-Davidson eventually made its presence known with its sturdy, strong and durable machines. In 1908, Walter Davidson, riding what came to be known as the Silent Gray Fellow, scored a perfect 1,000 points at the 7th Annual Federation of American Motorcyclists Endurance and Reliability Contest. Soon after, Walter Davidson, Arthur's brother, set the FAM economy record at 188.234 miles per gallon. By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world.