rconi coherer (filings), as this is
practically the only coherer from which so large a current can be obtained.

There have been many attempts made to receive with an ordinary filings
coherer, but as was pointed out in Chapter I. these have now been discarded
in serious wireless work, being only used in small amateur stations or
experimental sets. As the reasons for this are well known to the majority
of wireless workers there is no need to enumerate them here.

A method whereby a filings coherer can be decohered, the act of decohering
closing a local circuit which contains the photographic {41} receiving
apparatus, is given in the diagram Fig. 21.

[Illustration: FIG. 21.]

In the figure, the coherer C is fixed in rigid supports, one support being
provided with a platinum pin F. To the coherer is connected the sensitive
electro-magnet M, which becomes magnetised as soon as the incoming waves
act upon the coherer. To the armature B is attached a light aluminium arm
S, pivoted at K, and carrying at the other end the striker G, which is
fitted with a platinum contact. When the armature B is attracted the
coherer is decohered by the force of the impact between the contacts F and
G. To prevent damage to the coherer the force of the blow is taken off by
the ability of the striker to work back through a hole in the arm S, the
spring {42} N keeping it normally in a fixed position. T and P are
adjusting screws, and the terminals J are for connecting to the receiving
apparatus. With this arrangement a very short wave-train causes only one
tap of the contacts, so that only one mark is registered on the receiving
drum for every contact made on the transmitter.

[Illustration: FIG. 22.]

The drawing, Fig. 22, gives a diagrammatic representation of apparatus
arranged for another photographic method of receiving. The machine shown in
Fig. 6 is used in this case. A is the aerial, E earth, P primary of
oscillation-transformer, S secondary of transformer, C variable condenser,
C' block condenser, D detector, X two-way switch, T telephone.

A De' Arsonval galvanometer H is also connected to the switch X, so that
either the telephone or the galvanometer can be switched in. The {43}
galvanometer can be made sensitive enough to work with a current as small
as 10^{-7} of an ampere, with a period of about 1/150th of a second. The
screen J has a small hole about 1/8 inch diameter drilled in the centre.
Under the influence of the brief cu