As typical transmitter the Blake instrument may be cited. It is a carbon
microphone. It is shown in section in the cut; a is the mouthpiece and e
is a diaphragm of iron plate, although other substances could be used; f
is a steel spring, with a platinum contact piece at its end. One end
bears against the diaphragm, the other against a carbon block k. The
latter is carried by a brass block p, and pressure is maintained between
these contacts by the spring g and weight of the piece c, which by
gravity tends to press all together. The current passes by way of the
spring f, carbon button k and spring g through the circuit indicated.
A battery is in circuit with these parts. If a telephone is also in
circuit, and the transmitter is spoken against, the diaphragm vibrating
affects the resistance of the carbon-platinum contact, without even
breaking the contact, and the telephone reproduces the sound. The heavy
piece of metal C acts by its inertia to prevent breaking of the contact.
The position of this piece c, which is carried by the brass plate m, is
adjusted by the screw n.
Fig. 326. SECTION OF BLAKE TRANSMITTER.
In practice the transmitter and battery are usually on a local circuit,
which includes the primary of an induction coil. The line and distant
receiving telephone are in circuit with the secondary of the induction
coil, without any battery.
Telephone, Bi-.
A pair of telephones carried at the ends of a curved bar or spring so
that they fit the head of a person using them. One telephone is held
against each ear without the use of the hands.