Hello, operator, this question went to our "phoney" scientist Mark B. and he gave us this answer.

The modern telephone is a device that allows us to talk and listen to people anywhere in the world. Before the telephone there was the TELEGRAPH in which the sender using a switch coded his message into electrical bursts that represented the alphabet, the receiving operator, hearing and understanding the code could reconstruct the bursts into words and sentences. The SEMAPHORE was the earliest encoding method and MORSE CODE being the most popular. (SOS is ... ---... pronounced dot dot dot dash dash dash dot dot dot). Before this, apart from shouting, sound was transmitted by such devices megaphones, speaker tubes, and the string telephone. All very limited in distance of transmission.

A Canadian, Alexander Graham Bell (born in Scotland), invented the telephone in 1876. The classic telephone works almost the same as the telegraph but instead of sending codes as electrical bursts the telephone converts the human voice sound into electrical signals which travel through interconnecting wires to a second telephone were the electrical signal is reconstructed into the sound of the human voice.

The Transmitter

When you speak into a phone you are speaking into the TRANSMITTER. Typically this is a carbon microphone (invented by Thomas Edison). The carbon microphone is a type of TRANSDUCER whose purpose is to turn sound energy, your voice, into electrical energy, to travel down wires. The carbon microphone uses a thin piece of metal called a diaphram which vibrates as you speak. Behind the diaphram are tiny grains of carbon, like grains of sand. The vibrations from the diaphram wiggle the grains around and pack them loosely or tightly depending on how much vibration your voice makes. Electricity flows through these grains. When the grains are loosely packed, only a small amount electricity can get through (like a water tap almost off). When the grains are tightly packed, a lot of electricity can get through (like a water tap all the way on). The different amounts of electricity flowing through the grains are copies or representations of the different sounds of your voice.

Through the Wires

Now as the grains in your transmitter are wiggling at your end of the line, the different amounts of electricity travel through the wires to the telephone on the other end.

The Receiver

The person your talking to has the RECEIVER of his telephone to his ear to listen to you. Inside the receiver is a SPEAKER which has another diaphram. A speaker is the opposite transducer to the microphone. The speaker turns electrical energy into sound energy. Connected to the diaphram is an ELECTROMAGNET (a magnet controlled by electricity.) When the electricity from your transmitter reaches the receiver of the person your talking to, the electricity makes the electromagnet pull the diaphragm toward it. A small amount of electricity moves the magnet a small amount. A large amount of electricity moves the magnet a large amount. The large and small pulls make the diaphram vibrate. This vibration of the diaphram makes the air in front of it move creating sound waves. The different sound waves are copies of the different electricity and as we said earlier the different amounts of electricity are copies of the different sounds of your voice. Therefore the person your talking to doesn't here you voice but a copy of the sound waves of you voice.

Below is a picture of a telephone transmitter and telephone receiver.

Dialing Your Number? or Pushing Your Number?

When you are ready to make your call you pick up the phone which connects you to a local exchange or switch network that when instructed by your number will connect to the phone you are dialing. You then dial your number and an electrical connection is made between your phone and the one you are calling.

Older phones had a circular dial whose position around the dial represented a single number 0 to 9 hence the term to "dial" a number. Most modern phones use push buttons, one for each number. Some locations use standard dial and some use touch tone. The old dial phones would mechanically generate the required number of bursts of electrify to represent each number dialed. The local exchange understands these bursts of electricity and finds a path to the person your calling. The touch tone phone uses individual audio tones to represent the numbers dialed, this local exchange understands these tones and finds a path to the number your calling. These audio tones are similar to a music scale and very from low tones to higher tones.

When push button phones were first introduced and to keep phones and phone systems workable together push button phones could work with either touch tone or dial systems using digi pulse, where by pressing a button on the phone would causes the phone to generate electronically a dial pulse type signal in digi pulse mode or a tone signal in touch tone mode.

Check out the encyclopedia, Growing up with Science volume 18 page 1602, for more information about telephones. It's in your school library.

Thanks Mark. If you look on the Science board in your school library, you will see the insides of a modern telephone. Keep checking the Science corner, we are working on building a telephone system that you can try.

Thanks for your questions.

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