stance 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.]

We are now in a position to examine the apparatus of which a coherer
forms part (Fig. 61). First, we notice the aerial 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, and operates
the magnet M as well as the _tapper_, which is practically an electric
bell minus the gong. (The tapper circuit is indicated by the dotted
lines.)

We will suppose the transmitter of a distant station to be at work. The
electric w