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��Popular Science Monthly
���Although the submarine is bHnd after it dives it can be made to hear with the aid of micro- phones. If two hostile submarines were equipped so that they could hear each other there is no reason why they should not fight under water. Torpedoes would be the weapons used — torpedoes directed solely by the sound emanating from the craft to be destroyed
��marine could locate his prey by sound; suppose that he could hear a ship and locate her by sound more accurately, for example, than a blind man can locate the position of a ticking clock in a room? Might not that solve the problem?
With this thought in mind, I have worked out a method of utilizing micro- phones — a method which is a modifica- tion and extension of that which I described in the Popular Science Monthly for October, 191 5. Those who read that article will remember that I showed how it was possible to make a torpedo guide itself toward the beating propellers of a ship with the aid of microphones — "electrical ears," as I call them. A microphone is found in every telephone transmitter. It is an instrument for intensifying feeble sounds, or for transmitting sounds, and it is based on the principle that the transi- tion between loosely joined electric con- ductors decreases in proportion as they are pressed together. The conductors form part of a circuit through which a
��current is passing, and the variations in pressure due to sound waves in the vicinity of the conductors produce variations of resistance, and hence fluctuations of the current, so that the sounds are reproduced in a telephone receiver. In the modern telephone the transmitter is essentially a microphone, the pressure of the sound waves being communicated to the conductors by means of a diaphragm.
In a torpedo of the type I described in the Popular Science Monthly, the microphones are mounted in pairs on both sides of the nose. So long as the sound of the hostile ship's beating pro- pellers, traveling through water far more readily than sounds travel through air, affect all microphones with equal intensity, the torpedo rushes on straight to its mark. But if the vessel should change its course, the vibrations of the propellers would no longer strike the two pairs of microphones with equal force; one pair would be more affected than the other — the pair directly ex-
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