rbon brackets to one pole
of the battery, and another wire is led from the other bracket to one
terminal screw of the telephone, and the circuit is completed by a
wire from the other terminal of the telephone to the other pole of the
battery. If now the slightest mechanical jar be given to the wooden
frame of the microphone, to the table, or even to the walls of the room
in which the experiment takes place, a corresponding noise will be
heard in the microphone. By this delicate arrangement we can play the
eavesdropper on those insensible vibrations in the midst of which
we exist. 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. The contact between
the rod and the block in this 'hammer-and-anvil' form is, of course, the
portion which is sensitive to sound.

The microphone is a discovery as well as an invention, and the true
explanation of its action is as yet merely an hypothesis. It is supposed
that the vibrations put the carbons in a tremor and cause them to
approach more or less nearly, thus clo