Telephones

It was one of those moments when the world changes forever. On March 10, 1876, Thomas Watson was staring at a strange piece of mechanical apparatus when he heard it speak the words that made history: "Mr Watson! Come here! I want to see you!" Those three short exclamations mark the moment when the telephone properly came into being, thanks to Watson's brilliant colleague Alexander Graham Bell (1847-1922). Since that moment, a little over a century ago, the telephone has become one of the most commonplace inventions in the world—and has recently reinvented itself as the basic infrastructure on which the Internet is built.

Telephones seem quite simple, but what exactly are they and how do they route our calls round the world?

Last updated: January 12, 2007.

This article covers ordinary telephones or "landlines". If you want to find out about cellular (mobile) phones, take a look at our article on how cellphones work.

From telegraphs to telephones

Have you ever tried making a tin-can telephone out of two baked-bean cans and a length of string? It really does work! Not only that, it gives you a great insight into how a telephone carries people's voices from place to place. Normally, sounds travel through the air as invisible waves, transferring energy from something that vibrates (like a drum skin or a guitar string moving back and forth) to our eardrums. Sending sounds through the air is fine when the person we want to talk to is sitting in the same room. But if they're in another building—or even another country—we need a different form of communication.

Back in the 19th century, just before the telephone was invented, another piece of electrical equipment called the telegraph was the height of communication technology. A telegraph was a simple electrical circuit stretching many miles between two towns, typically alongside a railroad line. Messages could be sent back and forth down the telegraph line as bursts of electricity. How did that work? Think of a flashlight. It's a very basic electrical circuit: a loop of cable linking together a lamp, a switch, and a battery. Normally, when the light is off, the switch is set so it breaks the circuit. The switch is a kind of "drawbridge" that stops current flowing. When you flick the switch, you lower the drawbridge: you remove the break in the circuit, so electrons from the battery flow continuously around it, lighting the lamp.

Photo: A modern phone is almost entirely electronic. Look at all the circuits and chips packed inside!

Suppose you could make an absolutely gigantic flashlight hundreds of miles long. If you put the switch part at one end, say in New York City, and the lamp part at the other end, say in Detroit, you could send messages between those two places by flicking the switch on and off. You'd stand in New York City clicking the switch and someone else would stand in Detroit watching the light flash on and off. To send messages, you'd need to agree a special code beforehand so different kinds of flashes meant different things. If you wanted to be really clever, you could have two of these gigantic flashlights side-by-side, one to send messages from New York City to Detroit and the other to send messages back the other way. What you'd have built would have been a kind of telegraph. In real telegraphs, instead of a lamp, there's a device that makes a clicking noise at one end every time the switch (which is known as a key) is clicked on and off at the other end. And the people at the two ends use a prearranged pattern of short and long clicks ("dots" and "dashes") called Morse Code to send their information.

Telegraphs revolutionized communications, but they were slow and rather laborious to use. One of the main difficulties was that people had to learn Morse code before they could send and receive messages; another problem was that messages had to be sent and received a special telegraph offices: it was not possible to get them sent directly to your own home. Telephones changed all that.

A real telephone is like a cross between a baked-bean can telephone and a telegraph. When you "call" a friend on a baked-bean can telephone, you speak into the can at one end and the sound of your voice makes the can vibrate. The string then carries the vibrations to the can your friend is holding, which vibrates too, and produces sounds your friend can hear. Unlike a baked-bean can telephone, you can't speak into a telegraph. Instead, you send messages as coded pulses of electricity by flicking a switch on and off. Suppose you could combine these two ideas: what if you could turn the sound of your voice into an electrical signal that could be carried down a wire of any length, then turned back into a sound that someone else could hear at the other end? That was the idea that occurred to Alexander Graham Bell—and it's the principle behind a telephone.

Parts of a telephone

A telephone is not just the thing that sits on your table at home. It's a complete system: the handset at your end, the cable that runs into the wall, a whole collection of communication apparatus (copper cables, fiber-optics, microwave towers, and satellites) that carries telephone signals across country, some switching apparatus that makes sure calls get to the right place, and a handset at the other end. But let's think about the handset first.

Take a look at this photo of a typical phone handset. At the top, there's a loudspeaker you press against your ear. At the bottom, there's a microphone you put near your mouth. Coming out of the handset, wrapped inside a single thick, coiled cable, are two pairs of copper wires. One pair is an output: it takes outgoing electrical signals from the microphone to the telephone system; the other pair is an input: it takes incoming signals from the telephone system to the loudspeaker.

The loudspeaker and microphone work in similar but opposite ways. The microphone contains a flexible piece of plastic called a diaphragm with an iron coil attached to it and a magnet nearby. When you speak into the mouthpiece, the sound energy in your voice makes the diaphragm vibrate, moving the coil nearer to or further from the magnet. This generates an electric current in the coil that corresponds to the sound of your voice: if you talk loud, a big current is generated; if you talk softly, the current is smaller. You can think of a microphone as an energy converting device: it turns the sound energy in your voice into electrical energy. Something that converts energy from one form to another is called a transducer. The loudspeaker in a phone works in the opposite way: it takes an incoming electrical current and uses magnetism to convert the electrical energy back into sound energy you can hear. In some phones, the loudspeaker and microphone units are virtually identical, just wired up in opposite ways. (You can read more in our article about Loudspeakers and headphones.)

Photo caption: You can see the microphone really clearly when I unscrew the mouthpiece of this antique telephone. Note that the microphone is connected by just two wires: one carries electricity into the microphone; the other carries it back out again.

Making a call

Everyone knows what happens when you make a phone call: you pick up the handset, dial, and wait for the person at the other end to answer. But, just for a change, let's think about it from the phone's point of view.

1. Pick up the handset

When you pick up the handset, you switch on the telephone circuit: lifting the handset is effectively the same thing as flicking a switch that completes an electrical circuit between the handset and the local telephone exchange (a building full of telephone equipment in your local town or city that routes all the calls to and from your home).

2. Dial the number

One important part of a phone we've not mentioned yet is the push-button keypad. We still talk about "dialling" phone numbers even though hardly any phones (except antiques like the one in the first photo on this page) have rotary dials. On one of those old phones, you dial a number using a system called pulse dialling. If you listen to the handset as you dial, you hear lots of clicks going down the line as the dial rotates. Actually the dial is temporarily interrupting the electric current flowing down the line as it turns. The rapid pulses it generates in this way indicate to the local exchange what phone number you want to reach. A modern phone uses a different system called tone dialling (also known as DTMF, or dual-tone multi-frequency). As you press the numbers on the keypad, you hear musical notes going down the line instead of clicks. The exchange recognizes the number you want from the musical sounds your handset makes. (Tone dialling is also useful for things like telephone banking.)

3. Make the connection

You've picked up your handset and dialled the number. Now the exchange has to route your call to another phone in someone else's home. Imagine how this works. You can visualize an exchange as a huge building with thousands of wires coming into it from people's homes. If you wanted to connect Tom's phone to Ann's, in theory all you'd have to do would be to take the two cables leading to their homes and temporarily join them together. Back in the 19th century, when phones were still fairly new, this is pretty much what happened at the exchange. There was someone (typically a woman) called a switchboard operator. She would take one person's phone line and physically connect it to another by plugging it into a socket on a wooden board. She could connect any person's phone to anyone else's by switching around the connections on the board—which is why it was called a switchboard. Before long, these manual switchboards were replaced by electromagnetic ones that switched automatically. When transistors were invented in the late 1940s, switchboards became smaller, quicker, and more efficient. Today, switchboards are essentially just computers or digital exchanges that perform all the telephone routing automatically—but they still work essentially the same way as manual switchboards: they make a direct electrical connection from the handset in your home to the one in the home you are calling.

4. Talk into the phone

Once your call has been answered, you speak into the mouthpiece of your phone. Your voice generates sound energy when the vocal chords in your throat vibrate. The sound energy travels through the air into the microphone and makes the diaphragm inside vibrate. The diaphragm converts the energy from your voice into electricity, and this electrical energy flows down the phone line. When it reaches the handset at the other end, it flows into the loudspeaker in the earpiece. There, the electrical energy is converted back into sound—and your voice is magically recreated in the other person's ear. When the other person speaks, the entire process runs in reverse. Since there are wires running in both directions, you can both speak and listen at the same time.

To sum up what happens to energy when you use a phone to call a friend:

1. The sound energy in your voice makes the air vibrate. Vibrating air carries the sound energy into the phone.
2. The diaphragm in the mouthpiece microphone converts sound energy into electrical energy.
3. The electrical energy travels from the phone, via exchanges, to your friend's phone.
4. A diaphragm in the earpiece loudspeaker of your friend's phone converts the incoming electrical energy back to sound energy.
5. The sound energy travels out from the earpiece into your friend's ear.

Making a telephone call, then, is all about converting energy from sound to electricity, carrying the electricity down a very long wire, and then turning the electricity back into sound.

Transmitting calls

Once, all calls were carried down wires from one phone to another. That's why long-distance (sometimes called "trunk") calls took longer to route and were more expensive to make. International calls took so long to route that there was a very noticeable (and quite confusing) delay between you and the person at the other end, which was caused by the time it took for signals to travel down the wire. Now, calls travel in a whole variety of different ways. Most calls still go from homes to local exchanges along old-style copper wires. But calls can travel between exchanges down ultra-fast, high-capacity fiber-optic cables. Longer-distance calls are often beamed between urban centers using microwave towers (like small satellite dishes mounted on high buildings). International calls are typically bounced around the world using space satellites. Fiber-optics, microwave towers, and satellites send and receive phone calls not as electrical signals but as pulses of electromagnetic radiation (light or radio waves) travelling at the speed of light. That's why modern international phone calls are much faster, cheaper, and more reliable than they used to be—and why there's hardly any time lag on calls anymore.

Brief history

Although Alexander Graham Bell is generally credited with inventing telephones, his story is a controversial one. Bell became interested in sound and speech largely because his mother was deaf. Both his father and grandfather were noted experts on teaching deaf people and Bell too became a teacher of the deaf before making his name as an inventor. But other inventors were working on the idea of making a telephone at the same time as Bell. Elisha Graves (1835-1901), for example, filed a patent on a similar invention just hours after Bell made his own patent application. A third inventor, Antonio Meucci (1808-1889), seems to have developed the telephone in the 1840s—years before either Bell or Graves. In 2002, his contribution was finally recognized when the US Congress passed a bill in his honor.

Further Reading

Books you can read

Editors of Time and John Micklos, Jr. Alexander Graham Bell: Inventor of the Telephone. New York: HarperTrophy, 2006.

Favorite websites

• Brain Spin: Alexander Graham Bell: A light-hearted look at Mr Bell and his fabulous invention.
• The Alexander Graham Bell Family Papers: Check out the original history of the telephone at the US Library of Congress. You can read Bell's notebooks and look at his actual sketches of telephones!

Go shop

• Compare prices and read customer reviews of telephones at Amazon.com.
• UK: BT Shop: All kinds of phone and communications equipment, including home Phones, VoIP Phones, wireless home networking, home security, PC Hardware, components, software, digital cameras...
• UK: Compare prices and read customer reviews of home telephones at Kelkoo.