In wireless telegraphy we use the ether as the conductor of electrical disturbances.[13] Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and others have invented apparatus for causing disturbances of the requisite kind, and for detecting their presence. The main features of a wireless telegraphy outfit are shown in Figs. 59 and 61. THE TRANSMITTER APPARATUS. We will first consider the transmitting outfit (Fig. 59). It includes a battery, dispatching key, and an induction coil having its secondary circuit terminals connected with two wires, the one leading to an earth-plate, the other carried aloft on poles or suspended from a kite. In the large station at Poldhu, Cornwall, for transatlantic signalling, there are special wooden towers 215 feet high, between which the aerial wires hang. At their upper and lower ends respectively the earth and aerial wires terminate in brass balls separated by a gap. When the operator depresses the key the induction coil charges these balls and the wires attached thereto with high-tension electricity. As soon as the quantity collected exceeds the resistance of the air-gap, a discharge takes place between the balls, and the ether round the aerial wire is violently disturbed, and waves of electrical energy are propagated through it. The rapidity with which the discharges follow one another, and their travelling power, depends on the strength of the induction coil, the length of the air-gap, and the capacity of the wires.[14] [Illustration: FIG. 59.--Sketch of the transmitter of a wireless telegraphy outfit.] [Illustration: FIG. 60.--A Marconi coherer.] RECEIVING APPARATUS. The human body is quite insensitive to these etheric waves. We cannot feel, hear, or see them. But at the receiving station there is what may be called an "electric eye." Technically it is named a _coherer_. A Marconi coherer is seen in Fig. 60. Inside a small glass tube exhausted of air are two silver plugs, P P, carrying terminals, T T, projecting through the glass at both ends. A small gap separates the plugs at the centre, and this gap is partly filled with nickel-silver powder. If the terminals of the coherer are attached to those of a battery, practically no current will pass under ordinary conditions, as the particles of nickel-silver touch each other very lightly and make a "bad contact." But if the coherer is also attached to wires leading into the earth and air, and ether waves strike those wires, at every impact the particles will cohere--that is, pack tightly together--and allow battery current to pass. The property of cohesion of small conductive bodies when influenced by Hertzian waves was first noticed in 1874 by Professor D.E. Hughes while experimenting with a telephone. [Illustration: FIG. 61.--Sketch of the receiving apparatus in a wireless telegraphy outfit.]