8 HAYES— DETECTION OF SUBMARINES. 



intervening space is partially filled with fine carbon granules. One 

 plate is attached to a diaphragm which allows it to move back and 

 forth along a line perpendicular to its surface, thus compressing and 

 releasing the carbon granules. The cylindrical surface enclosing the 

 space between the graphite electrodes must be made a non-conductor. 



In operation the movable plate is rigidly attached to the inside of 

 a diaphragm enclosing a water-tight space. Two electrical leads pass 

 into the chamber through a water-tight stuffing-box and connect to 

 the two plates respectively. The motion of the diaphragm produced 

 by the sound waves causes corresponding changes in the pressure 

 between the carbon granules, since the inertia of the body of the 

 microphone is sufficient to prevent it from responding to the very 

 rapid sound vibrations of the diaphragm. The electrical resistance 

 through the microphone varies with the pressure between the car- 

 bon granules and therefore causes fluctuations in the electric cur- 

 rent when a battery is connected across the microphone leads. These 

 current fluctuations can be passed through a telephone and con- 

 verted into sound at the listener's ear. In practice the telephone 

 connection is made through a coupled circuit. Three types bf 

 coupled circuits which have been successfully used are shown in 

 Plate IX. 



The Magnetphone contains a movable coil of wire rigidly at- 

 tached to the inside of a diaphragm enclosing a water-tight space. 

 The leads from this coil pass from the chamber through a water- 

 tight stuffing-box. The movable coil is placed in a radial magnetic 

 field furnished by either a permanent or an electro-magnet. The 

 vibration of the diaphragm causes the coil to cut across the lines of 

 force, thereby generating electromotive forces which set up fluctu- 

 ating currents through the coil when electric connection is made be- 

 tween the coil leads. These fluctuating currents can be passed 

 through a telephone receiver and converted into sound at the lis- 

 tener's ear. 



The Electrostatic Condenser Detector consists of a flat strip of 

 metal, preferably aluminum, surrounded by a thin film of celluloid 

 and this in turn surrounded by a layer of tin-foil. The whole is en- 

 cased in rubber tubing, the ends of which are vulcanized so as to 

 make the condenser water-proof. One rubber insulated electric lead 



