The air vibrations set up by the voice make the diaphragm vibrate also, so that it moves backwards and forwards. These movements are infinitesimally small, but they are sufficient to affect the lines of force of the magnet to such an extent that rapidly alternating currents of varying degrees of strength are set up in the coil and sent along the line wire. On arriving at the receiver these currents pass through the coil and produce rapid variations in the strength of the magnet, so that instead of exerting a uniform attraction upon the iron diaphragm, the magnet pulls it with constantly varying force, and thus sets it vibrating. The air in front of the diaphragm now begins to vibrate, and the listener hears a reproduction of the words spoken into the transmitter. The way in which the fluctuations of the current make the second diaphragm vibrate exactly in accordance with the first is very remarkable, and it is important to notice that the listener does not hear the actual voice of the speaker, but a perfect reproduction of it; in fact, the second diaphragm speaks. The reader probably will be surprised to be told that the transmitter and the receiver of a magneto-electric telephone are respectively a dynamo and electric motor of minute proportions. We provide a dynamo with mechanical motion and it gives us electric current, and by sending this current through an electric motor we get mechanical motion back again. In the transmitter of the telephone just described, the mechanical motion is in the form of vibrations of the metal diaphragm, which set up currents of electricity in the coil of wire round the magnet, so that the transmitter is really a tiny dynamo driven by the voice. The receiver is provided with electric current from the transmitter, and it converts this into mechanical motion in the diaphragm, so that the receiver is a little electric motor.