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.
On aircraft it is clear that no earth connection is possible. The antenna in that case usually hangs
vertically down from the machine or airship. Under these conditions the valuable effect of the earth connection is of course lost. As will be remembered, the earth-connected apparatus sends forth waves which cling more or less to the neighbourhood of the earth's surface, while those from the non-earthed apparatus as used by aircraft tend to fly in all directions. The latter apparatus is in fact almost precisely similar to that which Hertz used in his first experiments. Hence the range is comparatively poor under these conditions, but it is good enough for very valuable work in warfare. Communication between airman and artillery by this means has revolutionized the handling of large guns in the field.
To save the airman from the accidental catching of his aerial wire in a tree or on a building there is sometimes fitted a contrivance of the nature of wire-cutters so that he can at any moment cut himself free from it.
So far we have dealt almost exclusively with the naval and aerial use of this wonderful invention. It is employed, though in a lesser degree, in land warfare. In such cases the aerial may be merely a wire thrown on to and caught up on a high tree. More elaborate devices are used, however, such as a high telescopic tower similar to the tall fire-escape ladders of the fire-brigades. Anyone who has seen the ladders rush up to a burning building and commence to erect themselves almost before they have stopped will realise how valuable such a machine must be for forming a temporary and easily movable wireless antenna. The power which causes the tall tower to
extend itself erect in a few seconds is compressed air carried in cylinders upon the machine, while the power which takes it from place to place is a petrol motor, and since the latter can be made to re-charge the storage cylinders it is clear that in it we have a marvellously convenient adjunct to the wireless apparatus.