Howstuffworks.com's "How Telephones Work"

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How Telephones Work
by Marshall Brain

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Although we take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. If you want to talk to someone, all that you have to do is pick up the phone and dial a few digits. Instantly you are connected to that person and you can have a two-way conversation with him or her. The telephone network extends worldwide, so you can reach nearly anyone on the planet. When you compare that to the state of the world just 100 years ago, when it might have taken up to several weeks to get a one-way written message to someone, you realize just how amazing the telephone is!

In this edition of How Stuff Works we will look at the telephone device that you have in your house, as well as the telephone network it connects to. You can also explore a number of links to get into things in incredible detail!

The Telephone
Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the 1920s, you could connect it to the wall jack in your house and it would work fine!

The very simplest working telephone would look like this inside:

As you can see, it only contains 3 parts and they are all simple:

A switch to connect and disconnect the phone from the network. This switch is generally called the hook switch. It connects when you lift the handset.
A speaker, which is generally a little 50 cent 8-ohm speaker of some sort
A microphone. In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from your voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone.

That's it! You can dial this simple phone by rapidly tapping the hook switch - all telephone switches still recognize "pulse dialing" like this. If you pick the phone up and rapidly tap the switch hook 4 times, the phone company's switch will understand that you have dialed a 4, for example.

The only problem with the phone shown above is that when you talk you will hear your voice through the speaker. Most people find that annoying, so any real phone contains a device called a duplex coil or something functionally equivalent to block the sound of your own voice from reaching your ear. A modern telephone also includes a bell so it can ring and a touch-tone keypad and frequency generator. A "real" phone looks like this:

Still, it's pretty simple! In a modern phone there is an electronic microphone, amplifier and circuit to replace the carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to generate a pleasant ringing tone. But a normal $6.95 telephone that you buy at Wal-Mart remains one of the simplest devices ever.

Go to the next article about How Telephones Work or pick from the table of contents:

The Telephone You are here
Creating Your Own Telephone Network
Calling Someone on the Telephone Network
Telephone Bandwidth
The Telephone Network
Links

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