a coherer, to hitch on, as it were, the stronger current of a local
battery. So the weak current of the ether waves, aided by the stronger
current of the local circuit, worked the recorder and wrote the message
down. The coherer was a little tube of glass not as long as your finger,
and smaller than a lead pencil, into each end of which was tightly
fitted plugs of silver; the plugs met within a small fraction of an inch
in the centre of the tube, and the very small space between the ends of
the plugs was filled with silver and nickel dust so fine as to be almost
as light as air. Though a small instrument, and more delicate than a
clinical thermometer, it loomed large in the working-out of wireless
telegraphy. One of the silver plugs of the coherer was connected to the
receiving wire, while the other was connected to the earth (grounded).
To one plug of the coherer also was joined one pole of the local
battery, while the other pole was in circuit with the other plug of the
coherer through the recording instrument. The fine dust-like silver and
nickel particles in the coherer possessed the quality of high
resistance, except when charged by the electric current of the ether
waves; then the particles of metal clung together, cohered, and allowed
of the passage of the ether waves' current and the strong current of the
local battery, which in turn actuated the Morse sounder and recorder.
The difficulty with this instrument was in the fact that the metal
particles continued to cohere, unless shaken apart, after the ether
waves' current was discontinued. So Marconi invented a little device
which was in circuit with the recorder and tapped the coherer tube with
a tiny mallet at just the right moment, causing the particles to
separate, or decohere, and so break the circuit and stop the local
battery current. As no wireless message could have been received without
the coherer, so no record or reading could have been made without the
young Italian's improvement.