that by the movement of a wire attached to a vibrating tympan along a
fixed wire conveying a current from a battery, and thereby shunting the
current at various positions along the length of the fixed wire, the
strength of the current in the derived circuit, in which was included a
suitable receiver, was varied accordingly. In other experiments mercury
was employed, either as a sliding-drop, inclosing the fixed wire, or as
an oscillating column; but these experiments, though instructive and
interesting, did not for various reasons give encouraging results with a
view to the practical application of the principle.

They, however, led Mr. Munro to proceed with compound wire structures,
such as gratings resting upon or rubbing against one another, and one of
the first experiments in this direction proved very successful, and led
Mr. Munro to the construction of his gauze telephone, which is the most
characteristic and efficient of his practical apparatus.

This instrument consists essentially of two pieces of iron-wire gauze,
the one fixed in a vertical plane, and the other resting more or less
lightly against it, the pressure between them being regulated by an
adjustable spring or weight. These gauze plates are so connected in a
telephonic circuit as to constitute the electrodes of a microphone; for
touching one another lightly in several points, they allow the current
to be transmitted between them in inverse proportion to the resistance
offered to it in its passage from one to the other. Under the influence
of sonorous vibrations the one plate dances more or less on the other,
thus varying the resistance; and very perfect articulation is produced
in a telephonic receiver included in the circuit. The gauze transmitter
so constructed may be fixed within a wall-box with or without a
mouthpiece; but as the sound waves acting directly upon the gauze plates
set them into agitation through their sympathetic vibration or by direct