It is easy to see that by passing an oscillating current, which is a very rapidly alternating current, through one of these one-direction conductors one half of each oscillation is more or less stopped. Oscillations, again, are surgings to and fro: the crystal tends to let the "tos" go through and to stop the "fros." That does not quite explain all that happens. It is not fully understood. The fact remains, however, that by putting a crystal in series with the telephone the oscillations become directly audible. The term "in series with" means that both crystal and telephone are inserted in the local receiving circuit so that the currents in that circuit pass through both in succession. The resistance of the crystal being very great, a special telephone is needed for use with it. It is quite an ordinary telephone, however, except in that it is wound with a great many turns of very fine wire and is therefore called a high-resistance telephone. Whichever of these detectors be used, then, the operator sits, with his telephone clipped on to his head, and with his tuner set for that wave length at which his station is scheduled to work, listening for signals. He may go for hours without being called up, and in the meantime he may hear many signals intended for others. He knows they are not for him, since every message is preceded by a code signal indicating to whom it is addressed. Under the conditions of warfare there is far more listening than there is sending, but when a station wishes to send the operator just switches over, cutting out his receiving apparatus and bringing his transmitting instruments into operation, and, having adjusted his tuner for the wave length of the station to which he desires to communicate, he flings out his message. In war-time, too, there is much listening for the signals of the enemy, which is the reason why as few messages are sent out as possible. In this case the man sits with his telephone on his head carefully changing his tuner from time to time in the endeavour to catch any message in any wave-length which may be travelling about. This searching the ether for a chance message of the enemy must be at times a very wearisome job, but it must be varied with very exciting intervals.