The weight of the crayon suffices to give the required pressure on the
contacts, both upper and lower, for the upper end of the Pencil should
lean against the inner wall of the cup in the upper bracket. The
brackets are fixed to an upright board of light, dry, resonant pine-
wood, let into a solid base of the same timber. The baseboard is with
advantage borne by four rounded india-rubber feet, which insulate it
from the table on which it may be placed. To connect the microphone up
for use, a small voltaic battery, say three cells (though a single cell
will give surprising results), and a Bell speaking telephone are
necessary. A wire is led from one of the carbon 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.
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