The Bell or Violin System.—This is a signaling system that is widely used on ocean going vessels of all kinds to send out warning signals in thick weather.
It is a system in which a large bell or other vibrating apparatus sets up sound waves which travel in every direction through the water; when these waves reach another craft they are heard by means of a telephone receiving apparatus.
Now, water will carry sound about ten times as far and four times as fast as air; that is to say, if a bell is struck a blow in air and the sound waves it sends forth can be heard a distance of half a mile, then the same bell if it is struck when it is submerged in water will send out waves to a distance of five miles.
The sending apparatus consists of either a bell (see [Fig. 57]), the striking mechanism of which is worked by electricity, or of a large saw-toothed wheel which revolves rapidly against a tight wire and in consequence sets up a musical note. This latter kind is called a violin transmitter.
Whichever is used is lowered through a hatch in the hull of the submarine, and the bell is rung or the wheel is rotated by an electro-mechanism, which sends out sound waves to distances of from 5 to 15 miles.
The receiving apparatus (shown in [Fig. 57]) is formed of an ordinary telephone transmitter, and this is fixed in a small iron tank filled with water and bolted to the inside of the skin of the hull. There is one of these transmitters on each side of the ship, and each one is connected with a battery and a pair of head telephone receivers placed in the navigating compartment.
FIG. 58. HOW THE SENDING AND RECEIVING BELL SYSTEM WORKS.
When the bell or violin of another submarine, or the base ship, sends out its message in the Morse alphabet the sound waves, as you will see by looking at [Fig. 58], travel through the water and strike the hull of the ship, go through its skin, set the water to vibrating in the tank, and this, acting on the telephone transmitter, makes it vary the electric current of the battery; the varying current flowing through the telephone receivers reproduces the distant sound of the bell or the violin wheel, and the operator on the submarine hears it.
The receiving apparatus also serves to detect the presence of an enemy ship when it comes within torpedoing range by the sound waves set up by and by which are sent out through the water by the rapid turning of the ship’s propellers.