The telephone uses a mouthpiece, composed of a transmitter, which is made up of a thin metal diaphragm. Inside the diaphragm is a small chamber filled with carbon granules. When we speak, sound waves cause the granules to compress, which allows a low-voltage electric current to pass.
Today, that current usually comes from a cell-phone battery. The electric current then travels. The louder we speak, the more the granules are compressed, and the heavier voltage of electric current is released. When the carbon grains are compressed, electric currents pass through them more easily. If we don't make any sound, no sound waves will cause the grains to compress, which would not allow electric currents to pass.
By copying the pattern and loudness of our voice, the release of the electric current through the carbon grains serves as the same pattern when the current hits the diaphragm on the other line. In other words, the vibrations caused by our voice are converted into electrical impulses that travel through wires until they reach the receiver. The receiver (ear-piece), which also has an iron diaphragm, converts the electrical impulses back into sound. Once the electric current hits the electromagnetic field in the receiver, the diaphragm vibrates and produces sound waves almost exactly like the pattern of sound waves at the point of origin.
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