Three French nails,
two parallel beneath and one laid across them, or better still a log-
hut of French nails, make a perfect transmitter of audible sounds, and a
good microphone. Because of its cheapness, its conducting power, and
its non-oxidisability, carbon is the most select material. A piece of
charcoal no bigger than a pin's head is quite sufficient to produce
articulate speech. Gas-carbon operates admirably, but the best carbon
is that known as willow-charcoal, used by artists in sketching, and when
this is impregnated with minute globules of mercury by heating it white-
hot and quenching it in liquid mercury, it is in a highly sensitive
microphonic condition. The same kind of charcoal permeated by platinum,
tin, zinc, or other unoxidisable metal is also very suitable; and it is
a significant fact that the most resonant woods, such as pine, poplar,
and willow, yield the charcoals best adapted for the microphone.
Professor Hughes' experimental apparatus is of an amusingly simple
description. He has no laboratory at home, and all his experiments were
made in the drawing-room. His first microphones were formed of bits of
carbon and scraps of metal, mounted on slips of match-boxes by means of
sealing-wax; and the resonance pipes on which they were placed to
reinforce the effect of minute sounds, were nothing more than children's
toy money boxes, price one halfpenny, having one of the ends knocked
out.
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