light, should not be loose."

[Illustration: Fig. 2.]

A writer, who signs "W.E.H.," in _The Electrician_ of 24th February
last, says: "The variation of current arises from a variation of
conductivity between the electrodes, consequent upon the variation of
the closeness or pressure of contact;" and also, "there must be a
variation of pressure between the electrodes when the transmitter is in
action."

It seems, then, that some scientific men agree that variation of
pressure is required to produce action in a microphone, and some of them
admit that a microphone with loose contacts will transmit articulate
speech, while others deny it, and some admit that a jolting or shaking
motion of the parts of the microphone does not interfere with articulate
speech, while others say such motion would break the circuit, and cannot
be relied on.

I will now describe two microphones in which there is a shaking or
jolting motion, and loose contacts, and no variation of pressure of the
carbons against one another, and both of these microphones when used
with an induction coil and battery give most excellent articulation. One
of these microphones is made as follows: Two flat plates of carbon are
secured to a block of cork, insulated from each other; into a hole of
each carbon a pin of carbon fits loosely, projecting above the carbons;
another flat piece of carbon, having two holes in it, bridges over the
two lower carbons, being kept in its place by the pins of carbon which
fit loosely in the holes in it, the bottom carbons being connected with
the battery; a block of cork has a flat side of it cut out so as when