You are here: Home page > Communications > Telephones
Advertisement

Modern large-button telephone

Telephones

by Chris Woodford. Last updated: January 14, 2014.

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. Apart from handling voice calls, it helps us send documents 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: This modern push-button phone has various extra features to help people with hearing impairments, including extra amplification (on both the loudspeaker and the microphone) and a bright flashing strobe light across the bottom to catch your attention if you can't hear the ring. This model also has giant buttons to help people with visual impairments.

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.

A cordless telephone being held in the hand with its antenna extended

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.

Photo: 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.

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.

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.

The microphone on an antique dial telephone

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 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.

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.)

What's inside an antique telephone?

They don't make them like they used to! Take the lid off an antique phone and here's what you find:

Photo showing the inside workings of an antique dial telephone

  1. Dial mechanism: as the dial rotates, it interrupts the circuit between the phone and the exchange creating dialing pulses. If you dial number "9" you create nine pulses, dial "5" and you make five pulses, and so on.
  2. Bells: Old phones actually had real, shiny metal bells inside them to indicate an incoming call! Modern phones have electronic buzzers or bleepers, which don't sound nearly so nice.
  3. Electromagnet: When the phone rings, this electrically controlled magnet switches on and off rapidly, moving a clapper in between the bells at the front and making them ring. It's great to watch this happening when you take the cover off. It's like being back in the 19th century!
  4. Tilting switch mechanism: This detects when the handset is lifted or put back. If the handset rests on the switch, it cuts off the connection between the phone and the local exchange. The circuit is opened only when you lift the handset.
  5. Circuit board: All the little wires that control different parts of the phone (the dialing, the tilt switch, and so on) meet on this circuit board.

Photo on an antique British GPO dial phone.

Where can I buy an antique phone?

You can still find phones like this British GPO (Post Office) model from the 1970s, for example on eBay, but there are lots of fake antique phones around—so be careful.

The pulse dialing used by some old phones no longer works on some digital exchanges, so although phones like this will take incoming calls, they don't always dial outgoing ones. They look great and sound fantastic when they ring, but the loudspeaker and microphone are quite crude compared to those in more modern phones and you may have trouble using one of these if you're hard of hearing.

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 "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.)

3. Make the connection

A typical female telephone switchboard operator.

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. Until about the mid-20th 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.

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

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.

Artwork showing the stages in a telephone call and how energy is transformed at each point

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 (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.

Who really invented the telephone?

Alexander Graham Bell

Although Alexander Graham Bell (1847–1922) 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 Gray (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 Gray. In 2002, his contribution was finally recognized when the US Congress passed a bill in his honor.

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

Bell's patent

Alexander Graham Bell's telephone patent from 1876, US patent #174465.

Bell's patent ("Telegraphy," US patent number 174,465) was filed on February 14, 1876 and granted the following month, 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. 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 near a 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.

Find out more

You can read Bell's patent for yourself by going to the US Patent and Trademark Office website and searching for patent 174,465; alternatively, you can find it at Google Patents.

Find out more

On this website

Books for younger readers

Books for older readers

Technical books for older students

History articles

Websites

Sponsored links

Please do NOT copy our articles onto blogs and other websites

Text copyright © Chris Woodford 2007, 2012. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Share this page

Press CTRL + D to bookmark this page for later or tell your friends about it with:

Cite this page

Woodford, Chris. (2007) Telephones. Retrieved from http://www.explainthatstuff.com/telephone.html. [Accessed (Insert date here)]

More to explore on our website...

Back to top