aves strike the aerial wire of the receiving station, and
cause the coherer to cohere and pass current. The relay is closed, and
both tapper and Morse inker begin to work. The tapper keeps striking the
coherer and shakes the particles loose after every cohesion. If this
were not done the current of A would pass continuously after cohesion
had once taken place. When the key of the transmitter is pressed down,
the waves follow one another very quickly, and the acquired conductivity
of the coherer is only momentarily destroyed by the tap of the hammer.
During the impression of a dot by the Morse inker, contact is made and
broken repeatedly; but as the armature of the inker is heavy and slow to
move it does not vibrate in time with the relay and tapper. Therefore
the Morse instrument reproduces in dots and dashes the short and long
depressions of the key at the transmitting station, while the tapper
works rapidly in time with the relay. The Morse inker is shown
diagrammatically. While current passes through M the armature is pulled
towards it, the end P, carrying an inked wheel, rises, and a mark is
made on the tape W, which is moved continuously being drawn forward off
reel R by the clockwork--or electrically-driven rollers R^1 R^2.

SYNTONIC TRANSMISSION.

If a number of transmitting stations are sending out messages
simultaneously, a jumble of signals would affect all the receivers
round, unless some method were employed for rendering a receiver
sensitive only to the waves intended to influence it. Also, if
distinction were impossible, even with one transmitter in action its
message might go to undesired stations.

There are various ways of "tuning" receivers and transmitters, but the
principle underlying them all is analogous to that of mechanical
vibration. If a weight is suspended from the end of a spiral spring, and
given an upward blow, it bobs up and down a certain number of times per
minute, every movement from start to finish having exactly the same
duration as the rest. The resistance of the air and the internal
friction of the spring gradually lessen the amplitude of the movements,
and the weight finally comes to rest. Suppose that the weight scales 30
lbs., and that it naturally bobs twenty times a minute. If you now take
a feather and give it a push every three seconds you can coax it into
vigorous motion, assuming that every push catches it exactly on the
rebound. The same effect would be produced more s