The oscillator consists of a ring-shaped electro magnet within which is a steel cylinder or core upon which is wound the requisite number of turns of wire. In the air gap between the core and the electro-magnet is a copper tube. The supports of the copper tube are two steel discs, rigidly clamped to a shaft, one end of which is firmly secured to a steel diaphragm, the outer face of which is in contact with the water when the oscillator is in use. The field magnet is energized by a direct current when the oscillator is in operation either for sending or receiving. When used for send ing, an alternating current of approxi mately 540 cycles per second is made to pass through the core winding. This causes cur rents of electricity to be generated by induc tion in the copper tube with the result that great forces are produced which cause the tube to move back and forth at the frequency of the alternating current. The motion of the tube is transmitted to the diaphragm and to the water in contact with it. When the oscillator is used for receiving, the core winding is con nected to telephone receivers. When incoming sound waves actuate the diaphragm, motion is imparted to the copper tube and currents of electricity are induced into the core winding. The currents in the telephone receivers, con nected to the core winding, produce sounds of the same periodicity as those vibrating the diaphragm of the oscillator.
The Fessenden oscillator, when sending, duces a series of waves of constant frequency or pitch so long as an alternating current i is passed through its core winding. By apply ing the alterating current to the oscillator for different lengths of time, it is possible to send series of waves of greater or less duration so that such series will correspond to the dots and dashes used in telegraphic communication.
Direction Finding.—It is customary when submarine signaling devices are used on ship board to use at least two instruments and to so mount them that sound which reaches one instrument may be screened, under certain con ditions, from the other. This is accomplished,
in the simplest case, by mounting two small tanks on the ship's plates, inside the ship, one on each side toward the bow. The tanks are filled with water
While the above arrangement is that which has been most commonly used in direction find ing, other methods are employed with satis factory results which do not require the ship's course to be changed in order to obtain direction.