The electro-
magnet was excited by the main line current and the armature attracted
to its poles at every signal, thus pressing on the plumbago, and by
reducing its resistance varying the current in the local circuit.
According as the main line current was strong or weak, the pressure on
the plumbago was more or less, and the current in the local circuit
strong or weak. Hence the signals of the local receiver were in
accordance with the currents in the main line.
Edison found that the same property might be applied to regulate the
strength of a current in conformity with the vibrations of the voice,
and after a great number of experiments produced his 'carbon
transmitter.' Plumbago in powder, in sticks, or rubbed on fibres and
sheets of silk, were tried as the sensitive material, but finally
abandoned in favour of a small cake or wafer of compressed lamp-black,
obtained from the smoke of burning oil, such as benzolene or rigolene.
This was the celebrated 'carbon button,' which on being placed between
two platinum discs by way of contact, and traversed by the electric
current, was found to vary in resistance under the pressure of the sound
waves. The voice was concentrated upon it by means of a mouthpiece and
a diaphragm.
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