t
augmented the sound in a telephone circuit. If, for instance, a light
pin, or a nail (A, Fig. 85) should be used to connect the severed ends
of a wire (B), the sounds in the telephone not only would be louder, but
they would be more distinct, and the first instrument made practically,
to demonstrate this, is shown in Fig. 86.

[Illustration: _Fig. 86._ MICROPHONE]

[Illustration: _Fig. 87._ TRANSMITTER]

HOW TO MAKE A MICROPHONE.--This instrument has simply a base (A) of
wood, and near one end is a perpendicular sounding-board (B) of wood, to
one side of which is attached, by wax or otherwise, a pair of carbon
blocks (C, D). The lower carbon block (C) has a cup-shaped depression in
its upper side, and the upper block has a similar depression in its
lower side. A carbon pencil (E) is lightly held within these cups, so
that the lightest contact of the upper end of the pencil with the
carbon block, makes the instrument so sensitive that a fly, walking upon
the sounding-board, may be distinctly heard through the telephone which
is in the circuit.

MICROPHONE THE FATHER OF THE TRANSMITTER.--This instrument has been
greatly modified, and is now used as a transmitter, the latter thereby
taking the place of the pin (A), shown in Fig. 85.

AUTOMATIC CUT-OUTS FOR TELEPHONES.--In the operation of the telephone,
the great drawback originally was in inducing users of the lines to
replace or adjust their instruments carefully. When switches were used,
they would forget to throw them back, and all sorts of trouble resulted.

It was found necessary to provide an automatic means for throwing in and
cutting out an instrument, this being done by hanging the telephone on
the hook, so that the act merely of leaving the telephone made it
necessary, in replacing the instrument, to cut out the apparatus.

Before describing the circuiting required for these improvements, we
show, in Fig. 87, a section of a transmitter.

A cup-shaped case (A) is provided, made of some insulating material,
which has a diaphragm (B) secured at its open side. This diaphragm
carries the carbon pencil (C) on one side and from the blocks which
support the carbon pencil the wires run to binding posts on the case.
Of course the carbon supporting posts must be insulated from each other,
so the current will go through the carbon pencil (C).

COMPLETE CIRCUITING WITH TRANSMITTER.--In showing the circuiting (Fig.
88) it will not be possible to illustrate the box