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. 



* For the benefit of those who may be iiii- 

 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. 



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- 



