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.
We are now in a position to examine the apparatus of which a coherer forms part (Fig. 61). First, we notice the aërial and earth wires, to which are attached other wires from battery A. This battery circuit passes round the relay magnet R and through two choking coils, whose function is to prevent the Hertzian waves entering the battery. The relay, when energized, brings contact D against E and closes the circuit of battery B, which is much more powerful than battery A,