Telephones

by Chris Woodford. Last updated: July 29, 2023.

It was one of those moments when the world changes forever. On March 10, 1876, Thomas Watson was staring at a strange piece of electrical 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. Apart from handling voice calls, it allowed documents to be sent by fax and it's also 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? Let's take a closer look!

Photo: Telephones used to be cumbersome and expensive pieces of equipment and, until mobile cellphones became popular from the 1980s, were invariably fixed in position. When you made a telephone call, the number you dialled reached a specific place; often you had to ask to speak to the person you wanted and wait while they walked to the phone. Mobile phones turned all of this on its head. Now phone numbers are linked not to places but to people, who take their phones and phone numbers wherever they go, even from one country to another.

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.

Artwork: Tin can telephones send sounds as vibrations down the string that connects them. The string has to be much tighter than I've drawn it here!

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.

Artwork: Samuel Morse demonstrates his telegraph: essentially, an electric circuit you open or close to send messages coded as pulses of electricity. Artwork from Frank Leslie's Illustrated Newspaper, 1871, courtesy of Library of Congress, Prints and Photographs Division.

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 replies 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 at special telegraph offices: it was not possible to get them sent directly to your own home. Telephones changed all that.

Photo: Modern telephones scoop up their power from the phone line, batteries, or a plug-in adapter; before electric power became so convenient and widespread, telephone users had to generate some of their own power for a call using a built-in, hand-cranked generator called a magneto. This is an early example of a magneto phone in Buffalo Gap Historic Village, dating from the late 19th century. Photo from The Lyda Hill Texas Collection of Photographs in Carol M. Highsmith's America Project, Library of Congress, Prints and Photographs Division.

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.

Let's think about 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.

Photo: You can see the relatively simple loudspeaker (left) and microphone (right) really clearly when you unscrew the mouthpiece and earpiece of this antique telephone. Both are connected by just two wires: one carries electricity in; the other carries it back out again. Photo by Tony Duell published on Wikimedia Commons under a Creative Commons (CC BY 2.0) licence.

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 articles about loudspeakers and microphones.)

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). I'll explain what happens in a telephone exchange later on in this article.

2. Dial the number

One important part of a phone we've not mentioned yet is the push-button keypad. We still talk about "dialing" phone numbers even though hardly any phones (except antiques like the one described up above) have rotary dials. On one of those old phones, you dial a number using a system called pulse dialing. 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 dialing (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 dialing is also useful for things like telephone banking.)

Photo: Push-button keypad: A cordless telephone handset like this is like a cross between a landline and a cellphone. Like a cellphone, it uses radio waves to communicate with a base station plugged into a normal landline outlet. It has quite a low powered radio transmitter so it works only within a short range of your home and garden. You can see the wireless antenna extended in this photo. Push-button phones slowly began to replace rotary dial phones from the 1960s, following the development of tone dialing (DTMF). Older phones had dials that sent pulses of current down the line to the exchange instead. Pulse dialing was much slower and it was quite easy to dial the wrong number without realizing.

3. Make the connection

Photo: A female telephone operator sitting at a switchboard in 1965. Photo by Martin Brown courtesy of NASA Glenn Research Center (NASA-GRC).

You've picked up your handset and dialed 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. In the late-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 using relays. When transistors were invented in the late 1940s, switchboards started to become 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. But if you want to make a call to another country, a few more things are involved.

Making international 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 (arranged in what's called a twisted pair). 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) traveling 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.

What does a telephone exchange do?

I've mentioned telephone exchanges a few times in this article without actually saying what they do or how they go about it.

Suppose there are five people in your street and they all want telephones so they can chat to one another. If you've got some baked bean cans to hand, it's easy to wire them all up. Each person needs a connection to all of the others, but that means quite a tangled mess: four baked bean cans in each home and four lines stretching taut to the other buildings. It's not great, but we could live with it. Now suppose there are five thousand people in your village and they all want to talk to one another. Each house would need 4999 lines running into it! Or what about if 10 million people in a city wanted phone lines instead? Have you really got room for 10 million phone lines in your home? You can see that it all gets a bit much very quickly.

Artwork: What a telephone exchange does. Top: Without an exchange, we need a separate line linking every home to every other home: each of these nine homes needs eight incoming lines! Bottom: With a central exchange, each home needs only one line linking it to the exchange, which can route calls on to all the other homes.

That's where telephone exchanges come in. Instead of each person being connected to everyone else, they're all connected to their local exchange, and the exchanges themselves are connected together. In our first example, the four people would need only one line each—and the exchange would be able to connect any pair of them together; in our last example, the 10 million people would still need only one line each. But as the number of people increases, we need to add more local exchanges, and now we're not just connecting one person to another but one person, through their local exchange (and potentially a series of exchanges) to another local exchange, until we reach the receiver of the call. What we end up with is a kind of spider's web of interconnections quite like our modern Internet. There are no permanent connections between any one line and any other: just a series of circuits that can be switched about so they connect together to make calls. This technology is called circuit switching.

When there were relatively few people with telephones, it was easy enough to have operators at telephone exchanges who could manually keep track of the calls by plugging leads in and out of switchboards. But as the system rapidly scaled up in size, and people came to expect faster calls, automated telephone exchange equipment rapidly took over. Although you might think that telephones were invented before exchanges, you'd be wrong. Exchanges were invented for the telegraph some years before Bell patented his phone, so the basic idea of central switching offices that could connect places together to exchange electrical messages really came first.

Who invented the automated exchange?

Photo: One of the electromagnetic switching units in a typical Strowger telephone exchange, c.1924. Photo by Harris & Ewing courtesy of US Library of Congress.

What Bell did for the telephone, Almon B. Strowger (1839–1902) of Kansas City, Missouri did for the telephone exchange. Reputedly, he invented the automated exchange switch because he was working as an undertaker and couldn't understand why calls to his business weren't getting through; someone at the switchboard was sending them to a rival instead. There had to be a fairer way of handling calls, he figured, and promptly decided to automate the process. It was a good call—and earned him fame and fortune.

In his first exchange patent, US Patent 447,918: Automatic Telephone Exchange, granted March 10, 1891, Strowger described how a traditional switchboard could be replaced by rotating cylinders with lots of connection points on them, which could be turned automatically by electromagnets to hook one telephone circuit to about 100 others. This is called a step-by-step switch or Strowger switch. The method it uses, which is known as rotary dialing, explains why telephones themselves were fitted with rotating dials. As you dialed a number, it sent little pulses of electricity down the line to the exchange. This made a series of chattering Strowger switches rise up or rotate by a certain number of positions so your call was automatically routed to its destination. (You can watch a video of this happening in the links below.)

Artwork: How Almon Strowger's step-by-step switch worked. The incoming phone line from which the call is being made is connected to the blue part of the switch via the yellow stand. The outgoing phone lines (orange) are connected to the little holes in the red cylinder. When you dial a number, a series of electromagnets (green) operate levers that move the blue part of the switch up or down and rotate the red cylinder so many places to connect the incoming line to the appropriate outgoing circuit through the appropriate hole. A single Strowger switch can handle about 100 lines, but linking a series of switches together lets you connect any number. Artwork from US Patent 447,918: Automatic Telephone Exchange by Almon Strowger, courtesy of US Patent and Trademark Office.

Although calls are routed electronically these days along fiber-optic cables, Strowger's basic technology remained in widespread use for almost a century, from the 1890s until about the 1980s. Though relatively unknown compared to inventing giants like Edison, Morse, and Ford, Strowger was, nevertheless, one of the most important and influential inventors of the 20th century. We might not have the Internet without him; the circuit-switching technology he invented ultimately evolved into a very different way of sending information down lines, called packet switching, which is how you're managing to read these words now! (You can find out more about that in our article about how the Internet works.

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Find out more

On this website

• Cellphones (Mobile phones)
• Fax machines
• History of communication
• Radio
• Satellites
• Voice Over Internet Protocol (VOIP)

On other sites

• 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!

Books

For younger readers

• Alexander Graham Bell: Inventor of the Telephone by The Editors of Time and John Micklos, Jr. New York: HarperTrophy, 2006.
• How Does a Network Work? by Matt Anniss. New York: Gareth Stevens, 2014. This 48-page book helps young readers to understand telephones in the wider context of modern computer networks and the Internet. Best for ages 9–12 (and probably the upper end of that range).

For older readers

• Reluctant Genius: Alexander Graham Bell and the Passion for Invention by Charlotte Gray. New York: Hachette Book Group, 2006.
• The Telephone Gambit: Chasing Alexander Graham Bell's Secret by Seth Shulman. New York: W.W.Norton, 2008.

Technical books for older students

• Telecommunication Switching Systems and Networks by T. Viswanathan. PHI Learning, 2015.
• Telecommunication System Engineering by Roger L. Freeman. John Wiley and Sons, 2015.
• The Telephone and Telephone Exchanges: Their Invention and Development by John E. Kingsbury. Longman, 1915. A classic, easy-to-understand explanation of the electromechanical telephone system as it existed for much of the 20th century. You can browse the whole text for free online or download it in various formats.

History articles

• How your new automatic telephone will work by Raymond Francis Yates. Popular Science, November 1921. Introducing... telephones that let you dial your own number! A glimpse of the future from almost a century ago.
• They're still inventing the telephone by Hans Fantel. Popular Science, December 1965. A look at forthcoming innovations in telephones as they loomed on the horizon a half century ago!
• Dr. Bell, Inventor of Telephone, Dies: The New York Times, August 3, 1922. Bell's obituary from the Times archive.
• The New Bell Telephone: Scientific American, October 6, 1877. SciAm announces a new invention!

Videos

• AT&T Archives: The First Call: A 1930 film celebrating Bell's invention, dusted down and repackaged for YouTube with a new introduction by George Kupczak of the AT&T Archives and History Center.
• AT&T Archives: The Electrical Transmission of Speech: An early Bell silent movie explaining the workings of the telephone using simple animations, which demonstrate how electrical circuits can carry speech.
• AT&T Archives: The Step-by-Step Switch: A great little black-and-white video explaining how Strowger switches in a telephone exchange route calls from one telephone to another.