Electromagnetic devices are less sensitive as receivers than microphones and are con sequently less disturbed by the waves pro ducing water noises. On account of this lack of sensitiveness they have not been as com monly used as the microphone. On the other hand, electromagnetic devices have been made which can be mounted directly in the side of the ship, the diaphragm of the instrument re placing a portion of the skin of the ship which is cut away. Instruments of this kind ar ranged in this way have proved most efficient sound-receiving devices.
The earliest sound-producing device was a bell struck under water. Innumerable other means of producing sound have been suggested, tried and abandoned. Water sirens, subaqueous explosions and similar methods have never been brought to a degree of perfection which has made com mercial use possible. The submarine bell has remained up to the present time, with the ex ception of the Fessenden oscillator to be later described, the most serviceable form of sub marine sound-producing device.
The development of a mechanism by means of which a submarine bell can be operated commercially in all weathers and for long periods of time has been very difficult. The problem has, however, been solved and several forms of submarine bells are now made to meet service requirements. One form, shown in Fig. 2, uses the movements of the surface waves to give the power required to strike the bell. In this form the bell and striking mechanism are supported at the requisite depth from a buoy or float. A wing, platform or is hinged to the sides of the bell case containing the striking mechanism, and is so counterbalanced that, as the bell case rises and falls with the waves, the differences in the move ments of the bell case and vane cause a pow erful spring within the bell case to be com pressed when fully compressed, to be re leased. The released spring draws up the striker of the bell and causes a sharp blow to be struck. The bell mechanism is such that, whenever there is wave motion, the bell is operated at frequent intervals. Bells thus made can be kept in successful operation for con siderable periods of time without attention and without any expense for the power consumed. The complete bell mechanism is so designed that corrosion, especially from sea water, is reduced to a minimum and, as all of the parts within the bell are immersed in oil, the wear upon the moving parts is small.
This form of submarine bell is the dis tinctive feature of the art of submarine signal ing. The bell is supported from a buoy moored in places where there is no lightship or where it is practically impossible to obtain any other form of power with which to actuate the signaling device. The bell stroke is powerful enough to be heard at all times much farther than the uncertain signals that might be produced in the air by a mechanism operated by the energy derived from the motion of the ocean. The submarine bell can be used without requiring the attendance of an opera tor and in places where it is impossible to utilize wireless apparatus employing ether waves. Submarine bells of this type are to day located at many points of danger on the coasts of most maritime countries and the num ber of bells in service is increasing.
Another form, shown in Fig 3, of submarine bell has been designed for use from lightships. The blow of the striker upon the bell proper in these bells is produced by compressed air derived from an air compressor on the light ship. When signals are to be sent, the pneu matic bell is suspended from the side of the lightship and the compressed air is delivered to the bell through a hose. By a simple arrange ment, the compressed air can be admitted by the normal operation of a key or automatically. Thus code signals, giving the number of the lighthouse station, can he sent out during the time the bell is required for operation.
Other forms of submarine bells are avail able for special conditions. Thus for service on buoys in rivers or other places where there is insufficient surface movement of the water to operate the vanes of the bell of the type first described above, an electric bell is provided which is operated by electricity generated on shore and carried to the bell by a cable.
Another commercial form of sound-produc ing device for submarine work is the oscillator shown in Fig. 4, devised by Prof. Reginald A. Fessenden. The Fessenden oscillator is an electromagnetic mechanism which is capable of producing sound waves in water of very large amplitude and, at the same time, can be con trolled with ease. The Fessenden oscillator is shown in Fig. 4. A diagrammatic cross-sectional drawing of the oscillator is shown in Fig. 5.