ls; C,
suspending rod; D, base; F, F', dipping rods.]

The local circuit of the relay is completed by means of a copper dipper in
mercury, somewhat resembling an ordinary mercury break, but modified to
suit the present requirements. The arrangement will be seen from Fig. 39.
The whole of the {82} moving parts are made as light as possible, and for
this reason the rod C and the dippers F, F' should be made as short as
convenient. The containers H, H' are separate, of cast iron, and
rectangular in shape. The dipper is of very thin copper tube--an advantage
where alternating current is to be used--and is made adjustable for height
on the suspending rod C. The leg F is of such a length that permanent
contact is made with the mercury in the container H, while the leg F'
clears the surface of the mercury by about 1/4 inch, when the armature of
the relay is in its normal position. To prevent undue churning of the
mercury, which would necessarily take place if the dipper entered and left
the mercury at each movement of the armature, a pointed ebonite plug is
inserted in the end of the tube. This will be found to give good results at
a high speed, the mercury being practically undisturbed, and the production
of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and
to prevent arcing, the surface is covered with paraffin oil. If this is not
sufficient to prevent arcing a condenser should be shunted across the {83}
containers. The volume of mercury, and the area of the dippers, should be
sufficient to carry the current used for a considerable period without
heating up to any extent. An adjustable weight J is provided in order to
balance the armature and dipping rod.

The remaining transmitting apparatus consists of the battery D^2 and the
usual wireless apparatus. The double-pole two-way switch B' is to enable
the photo-telegraphic set to be switched out and the hand key W switched in
for ordinary signalling purposes. The battery D^2 should be about 12 volts.

RECEIVING APPARATUS

The wireless portion of the receiver is similar to that given in Fig. 22,
is of the usual syntonic type, and comprises an oscillation transformer, S
being the secondary, and P the primary; C' is a block condenser, and C a
variable condenser. The detector D is of the carborundum crystal or
electrolytic pattern. A two-way switch B is provided so that the relay U
can be switched out and the telephones J switched in for ordinary receiving