How does a telephone work?

               Telephones transmit speech messages along wires by means of electrical signals. Telephones were invented as long ago as 1876.The handset of a telephone includes a loudspeaker and a very small microphone, which contains granules of carbon. When you talk into the microphone the sound waves of your voice cause a metal diaphragm to vibrate, and it presses against the carbon granules. The vibrations vary depending on the sounds. They change the very small amount of current flowing out along the wires to the receiver of another telephone.

               When the electric current carrying the signals reaches the receiver handset, the same variations in the current run through an electromagnet. This causes another diaphragm to vibrate in the earpiece, accurate reproducing the sound of the speaker’s voice.

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