| Fig. 86. Microphone | 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[p. 120] 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[p. 121] 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 boxes, or casings, which receive the various instruments. For instance, the hook which carries the telephone or the receiver, is hinged within the transmitter box. The circuiting is all that it is intended to show.
