The latter instrument changes the high frequency currents, which are alternating, into an interrupted direct current; and these in turn energize the telephone receiver, with the result that the dots and dashes sent out by the sending operator are reproduced by the telephone receiver, when they are heard by the receiving operator who is listening in.
The tuning coil and condenser enable the operator to tune his receiving apparatus to the length of wave which the transmitting station is sending out, and this operation is called tuning in.
The wireless station of a submarine is usually located in the navigation compartment. Although the aerial is neither high nor long, messages can be sent to upwards of two hundred miles and received over much greater distances.
Wireless allows the submarine not only to keep in touch with its base but also to pick up and intercept messages from enemy ships, and though the operator may not be able to decipher them it is possible for him to determine in about what direction and at about what distance the ship is.
Another use to which wireless is put is signaling between submarines that are doing patrol duty at the same time but which are too far away from each other to use either flags or lights.
Wireless telegraphy cannot, however, be used when the submarine is under water, for water absorbs the electric waves in exactly the same way that it absorbs light waves. But taken all in all, wireless is by far the most important of all the signaling systems yet invented, and it is the only one by which messages can be sent and received by either day or night, over long distances and in any kind of weather.
Underwater Signaling Systems.—As I have said before, there are two kinds of signaling systems used by a submarine when it is under water.
While both systems leave much to be desired—for neither can begin to come up to wireless, either in ease of operation or in signaling range—as they are the only known means by which underwater communication is possible, there is nothing to do but to use them.
