How do telephones work? (2024)

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

It was one of those moments when theworld changes forever. On March 10, 1876, Thomas Watson was staring at a strangepiece of electrical apparatus when he heard it speak the words thatmade history: "Mr Watson! Come here! I want to see you!" Those three shortexclamations mark the moment when the telephoneproperly came into being, thanks to Watson's brilliant colleagueAlexander Graham Bell (1847–1922). Since that moment, a little over a centuryago, the telephone has become one of the most commonplaceinventions in the world. Apart from handling voice calls, it allowed documents to be sent by fax and it's also the basicinfrastructure on which the Internet is built.Telephones seem quite simple, but what exactly are they and how do they route our calls roundthe world? Let's take a closer look!

Photo: Telephones used to be cumbersome and expensive pieces of equipment and, untilmobile 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 numberswherever they go, even from one country to another.

From telegraphs to telephones

Have you ever triedmakinga tin-can telephoneout of two baked-bean cans and a length of string?It really does work! Not only that, it givesyou a great insight into how a telephone carries people's voices fromplace toplace. Normally, sounds travel through the air as invisible waves,transferring energy from something thatvibrates (like a drum skin or aguitar string moving back and forth) to our eardrums. Sending soundsthrough the air is fine whenthe person we want to talk to is sitting in the same room. But ifthey're in another building—or even another country—we need a differentform 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 telegraphwas the height of communication technology. A telegraph was a simpleelectrical circuit stretching many miles between two towns, typicallyalongside a railroad line. Messages could be sent back and forth downthe telegraph line as bursts of electricity.How did that work? Thinkof a flashlight. It's a very basic electrical circuit: a loop of cablelinking together a lamp, a switch, and a battery. Normally, when thelight is off, the switch is set so it breaks the circuit. The switch isa kind of "drawbridge" that stops current flowing. When you flickthe switch, you lower the drawbridge: you remove the break in thecircuit, so electrons from the battery flow continuously around it, lighting thelamp.

Artwork: Samuel Morse demonstrates his telegraph: essentially, an electric circuit you openor 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 ofmiles long. If you put the switch part at one end, say in NewYork City, and the lamp part at the other end, say in Detroit, youcouldsend messages between those two places by flicking the switch on andoff. You'd stand in New York City clicking the switch and someone elsewould stand in Detroit watching the light flash on and off. To sendmessages, you'd need to agree a special code beforehandso different kinds of flashes meant different things. If you wanted tobe reallyclever, you could have two of these gigantic flashlights side-by-side,one to send messages from New York City to Detroit and the other tosend replies back the other way. What you'd have built would have beena kind of telegraph. In real telegraphs, instead of a lamp, there's adevice that makes a clicking noise at one end every time the switch (which isknown as a key) is clicked on and off at theother end. And the people at the two ends use a prearranged pattern ofshort and long clicks ("dots" and "dashes") called MorseCode to send their information.

Telegraphs revolutionized communications, but they were slow andrather laborious to use. One of the main difficulties was that peoplehad to learn Morse code before they could send and receive messages;another problem was that messages had to be sent and receivedat special telegraph offices: it was not possible to get them sentdirectly 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 telephoneand a telegraph. When you "call" a friend on a baked-bean cantelephone, you speak into the can at one end and the sound of yourvoice makes the can vibrate. The string then carries the vibrations tothe can your friend is holding, which vibrates too, and produces soundsyour friend can hear. Unlike a baked-bean can telephone, you can'tspeak into a telegraph. Instead, yousend messages as coded pulses of electricity by flicking a switch onand off. Suppose you could combine these two ideas: what if youcould turn the sound of your voice into an electrical signal that couldbe carried down a wire of any length, then turned back into a soundthat someone else could hear at the other end? That was the idea thatoccurred to Alexander Graham Bell—and it's the principle behind atelephone.

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 runsinto the wall, a whole collection of communication apparatus (copper cables,fiber-optics, microwave towers, andsatellites) that carriestelephone signals across country, some switching apparatus that makessure 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 thehandset, wrapped insidea single thick, coiled cable, are two pairs of copper wires. One pairis an output: it takes outgoing electricalsignals from the microphone to the telephone system; the other pair isan input: it takes incoming signals from the telephone system to theloudspeaker.

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 carrieselectricity in; the other carries it back out again.Photo by Tony Duell published on Wikimedia Commonsunder a Creative Commons (CC BY 2.0) licence.

The loudspeaker and microphone work in similar but opposite ways.The microphone contains a flexible piece ofplastic called a diaphragm with an iron coil attached to it and amagnet nearby. When you speak into the mouthpiece, the sound energy inyour voice makes the diaphragm vibrate, moving the coil nearer to orfurther from the magnet. This generates an electric current in the coilthat corresponds to the sound of your voice: if you talk loud, a bigcurrent is generated; if you talk softly, the current is smaller. Youcan think of a microphone as an energy converting device: it turns thesound energy in your voice into electrical energy. Something thatconverts energy from one form to another is called a transducer.The loudspeaker in a phone works in theopposite way: it takes an incoming electrical current and usesmagnetism to convert the electrical energy back into sound energy youcan hear. In somephones, the loudspeaker and microphone units are virtually identical,just wired up in opposite ways. (You can read more in ourarticles about loudspeakers andmicrophones.)

Making a call

Everyone knows what happens when you make a phone call: you pick upthe 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 ofview.

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 switchthat completes an electrical circuit between the handset and the localtelephone exchange (a building full oftelephone 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 thepush-button keypad. We still talkabout "dialing" phone numbers even though hardly any phones (exceptantiques like the one described up above) have rotarydials. On one of those old phones, you dial a number using a systemcalled pulse dialing. If you listen to thehandset as you dial, you hear lots of clicks going down the line as thedial rotates. Actually the dial is temporarily interrupting theelectric current flowing down the line as it turns. The rapidpulses it generates in this way indicate to the local exchange whatphone number you want to reach. A modern phone uses a different systemcalled tone dialing (also known as DTMF, ordual-tone multi-frequency). As you press the numbers on the keypad, youhear musical notes going down the line instead of clicks. The exchangerecognizes the number you want from the musical sounds your handsetmakes. (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 ResearchCenter (NASA-GRC).

You've picked up your handset and dialed the number. Now theexchange has to route your call to another phone in someoneelse's home. Imagine how this works. You can visualize an exchange as ahuge building with thousands of wires coming into it from people'shomes. Ifyou wanted to connect Tom's phone to Ann's, in theory all you'd have todo would be to take the two cables leading to their homes andtemporarily join themtogether. 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 anotherby plugging it into a socket on a wooden board. She could connect anyperson's phone to anyone else's by switching around the connections onthe board—which is why it was called a switchboard. Before long, thesemanual switchboards were replaced by electromagnetic ones that switchedautomatically using relays.When transistorswere invented in the late 1940s,switchboards started to become smaller, quicker, and more efficient. Today,switchboards are essentially just computers or digitalexchanges that perform all the telephone routing automatically—but they still work essentially thesame way as manual switchboards: they make a direct electricalconnection from the handset in your home to the one in the home you arecalling.

4. Talk into the phone

Once your call has been answered, you speak into the mouthpiece ofyour phone. Your voice generates sound energy when the vocal chords inyour throat vibrate. The sound energy travels through the air into themicrophone and makes the diaphragm inside vibrate. The diaphragmconverts the energy from your voice into electricity, and thiselectrical energy flows down the phone line. When it reaches thehandset at the other end, it flows into the loudspeaker in theearpiece. There, the electrical energy is converted back into sound—andyour voice is magically recreated in the other person's ear.When the other person speaks, the entire process runs in reverse. Sincethereare wires running in both directions, you can both speak and listen atthe 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 fromsound to electricity, carrying the electricity down a very long wire, and thenturning the electricity back into sound. But if you want to make a callto 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 longerto route and were more expensive to make. International calls took solong to route that there was a very noticeable (and quite confusing)delay between you and the person at the other end, which was caused bythe time it took for signals to travel down the wire. Now, calls travelin a whole variety of different ways. Most calls still go from homes tolocal exchanges along old-style copper wires(arranged in what's called a twisted pair). But calls can travel between exchanges down ultra-fast, high-capacityfiber-optic cables. Longer-distancecalls are often beamed between urban centersusing microwave towers (like small satellite dishes mounted on highbuildings). International calls are typically bounced around the worldusing space satellites. Fiber-optics,microwave towers, and satellitessend and receive phone calls not as electrical signals but as pulses ofelectromagnetic radiation(light or radio waves) traveling at the speed of light. That's why modern international phone calls are muchfaster, cheaper, and more reliable than they used to be—and why there'shardly 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 whatthey do or how they go about it.

Suppose there are five people in your street and they all wanttelephones so they can chat to one another. If you've got some bakedbean cans to hand, it's easy to wire them all up. Each person needs aconnection 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 wecould live with it. Now suppose there are five thousand people inyour village and they all want to talk to one another. Each housewould need 4999 lines running into it! Or what about if 10 millionpeople in a city wanted phone lines instead? Have you really gotroom for 10 million phone lines in your home? You can see that it allgets 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 personbeing connected to everyone else, they're all connected to theirlocal exchange, and the exchanges themselves are connected together.In our first example, the four people would need only one lineeach—and the exchange would be able to connect any pair of themtogether; in our last example, the 10 million people would stillneed only one line each. But as the number of people increases, weneed to add more local exchanges, and now we're not just connectingone 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 ourmodern Internet. There are no permanent connections between any one line andany other: just a series of circuits that can be switched about so theyconnect together to make calls. This technology is called circuit switching.

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

Who invented the automated exchange?

Photo: One of the electromagnetic switching units in a typical Strowger telephoneexchange, 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 inventedthe automated exchange switch because he was working asan undertaker and couldn't understand why calls to his businessweren't getting through; someone at the switchboard was sendingthem to a rival instead. There had to be a fairer way of handlingcalls, 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 traditionalswitchboard could be replaced by rotating cylinders with lots of connectionpoints on them, which could be turned automatically byelectromagnets 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 telephonesthemselves 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 positionsso your call was automatically routed to its destination. (You can watch a videoof 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 Exchangeby Almon Strowger, courtesy of US Patent and Trademark Office.

Although calls are routed electronicallythese days along fiber-optic cables, Strowger's basic technology remained in widespread use for almost a century, from the 1890s until about the 1980s. Thoughrelatively unknown compared to inventing giants like Edison, Morse, and Ford,Strowger was, nevertheless, one of the most important and influential inventors ofthe 20th century. We might not have the Internet without him; the circuit-switching technology he invented ultimately evolved into a very differentway 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 thatin our article about how the Internet works.

Who really invented the telephone?

Photo: Alexander Graham Bell. Courtesy of US Library of Congress.

Although Alexander Graham Bell (1847–1922) is generally credited with inventingtelephones, his story is a controversial one. Bell became interested insound and speech largely because his mother was deaf. Both his fatherand grandfather were noted experts on teaching deaf people and Bell toobecame a teacher of the deaf before making his name as an inventor.

But other inventors were working on the idea of making a telephone atthe same time as Bell. Elisha Gray (1835–1901), for example, filed a patent on a similarinvention just hours after Bell made his own patent application. Athird inventor, Antonio Meucci (1808–1889), seems to have developed thetelephone in the 1840s—years before either Bell or Gray. In 2002, hiscontribution was finally recognized when the US Congress passed a billin his honor.

Bell's patent

Bell's patent (US Patent 174,465: Telegraphy) was filed on February 14, 1876 and granted about three weeks later (March 7, 1876) and it describes various improvements to simple telegraphy (the method of sending messages down a length of wire using electric currents made famous by Samuel Morse). The most interesting part for modern readers is figure 7, shown here, in which Bell explains how his equipment can carry signals from "the human voice or by means of a musical instrument" (in other words, how his telegraph can be used as a telephone). You'll recognize how it works straight away if you've read the rest of this article.

Artwork: Alexander Graham Bell's original telephone patent. Courtesy of US Patent and Trademark Office.

Briefly:

1. The speaker talks into a horn.
2. The sound of their voice makes a diaphragm (a kind of small tight drum skin stretched across the narrow end of the horn) vibrate.
3. The vibrations move a coil near a magnet, converting the mechanical sound energy into a fluctuating electric current.
4. The electric current travels down a wire, which can (in theory) be any length.
5. At the receiving end, similar equipment reverses the process. The electric current flows into a coil placed neara magnet, making the coil move back and forth, and pushing another diaphragm.
6. The diaphragm, stretched over a second horn, recreates the original sound. The narrowing shape of the diaphragm helps to amplify the sound.

Scientific American announced "Professor Graham Bell's telephone" in a front-page article on October 6, 1877, noting, in particular, the "simplicity of the construction,"and sketching out some speculative applications that never came to pass:

"Perhaps, in the future, operatic or concert companies and lecturers, instead of trav­eling over the country, will simply send out telephonesenough to present each person of their audience in a distantcity with an instrument apiece, and do their talking andsinging once for all in the metropolis."

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.