If a feather or a camel-hair pencil be stroked along the base-
board, we hear a harsh grating sound; if a pin be laid upon it, we hear
a blow like a blacksmith's hammer; and, more astonishing than all, if a
fly walk across it we hear it tramping like a charger, and even its
peculiar cry, which has been likened, with some allowance for
imagination, to the snorting of an elephant. Moreover it should not be
forgotten that the wires connecting up the telephone may be lengthened
to any desired extent, so that, in the words of Professor Hughes, 'the
beating of a pulse, the tick of a watch, the tramp of a fly can then be
heard at least a hundred miles from the source of sound.' If we whisper
or speak distinctly in a monotone to the pencil, our words will be heard
in the telephone; but with this defect, that the TIMBRE or quality is,
in this particular form of the instrument, apt to be lost, making it
difficult to recognise the speaker's voice. But although a single
pencil microphone will under favourable circumstances transmit these
varied sounds, the best effect for each kind of sound is obtained by one
specially adjusted. There is one pressure best adapted for minute
sounds, another for speech, and a third for louder sounds. A simple
spring arrangement for adjusting the pressure of the contacts is
therefore an advantage, and it can easily be applied to a microphone
formed of a small rod of carbon pivoted at its middle, with one end
resting on a block or anvil of carbon underneath.
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