Page:Popular Science Monthly Volume 89.djvu/18

 Popular Science Monthly

��seeking to enter a liarhor at night or in a dense fog. In my system the mines arc arranged in groups of four, each group constituting a field unit. On each mine a microphone is mounted.* These micropliones Htcrally hear the hum of a submarine's motor. Not only that but the particular microphone which hears the submarine best, because it is the

��The field units are interconnected electrically, so that the entire harbor is sown not only with charges of high explosive but with microphone detectors. Interconnection is necessary because mines i and 2 of one group constitute mines 3 and 4 of an adjacent group. Microphones are so remarkably sensitive (they have picked up the hum of sub-

���As a submarine progresses through a minefield in the effort to reach shipping in a harbor, the hum of its electric motor is heard by microphones on the mines. The vibrations picked up by the microphones are electrically transmitted to shore and converted into visual signals by incandescent lamps corresponding in number and position with the microphoned mines

��nearest to it, can easily be located. It is simple enough to determine whether a submarine is nearer mine i or mine 3 of a given field unit of four mines.

f.imili.ir with the microphone 1 may stale that the iiiiirophonc is an instninicul for intensifying fccl)lc sounds or for Ir.insniilliiig sounds anil it is based on the principle (hat the transition lielween looscly-joincd electrical conductors decreases in proportion as they are pressed to- gether. The conductors form part of a circuit through which a current is passing, and the variations in pressure due to sound waves in the viiinity of the conductors produce variations of resistance and lluctuations of the current so that the sounds are reproduced in a telephone receiver. In the modern telephone I lie Iransmitler is essentially a microphone, the pressure of the sounil waves being connuunicated to the conductors by means of a diaphragm.
 * For the benefit of those who may be iiii-

��marines fifteen miles away in the present war) that they need not be la\ ishly employed in every case. Four micro- phones placed in the four corners of a small field would answer in many cases, all the more so, since a ship can be blown up even though it be fifty feet from the actual explosion. Few of us realize how terrific is the disriiplixe effect of the gases suddenly generated when se\eral lumdred pounds of explo- sive are detonated.

A mine-field efjuipped wiili micro- phones in the manner indie. ited is electrically coimected with a luminous .innimciator. In other words, wires rim from each microphone to a board which isdixided into s(|u. ires correspond-

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