This instrument was ordinarily mounted in a wooden
This instrument was ordinarily mounted in a wooden box together with
the induction coil, which is shown in the upper portion of the figure.
The Blake transmitter has passed almost entirely out of use in this
country, being superseded by the various forms of granular
instruments, which, while much more powerful, are not perhaps capable
of producing quite such clear and distinct articulation.
The great trouble with the single-contact transmitters, such as the
Blake, was that it was impossible to pass enough current through the
single point of contact to secure the desired power of transmission
without overheating the contact. If too much current is sent through
such transmitters, an undue amount of heat is generated at the point
of contact and a vibration is set up which causes a peculiar humming
or squealing sound which interferes with the transmission of other
sounds.
Multiple Electrode. To remedy this difficulty the so-called
multiple-electrode transmitter was brought out. This took a very great
number of forms, of which the one shown in Fig. 39 is typical. The
diaphragm shown at _1_, in this particular form, was made of thin pine
wood. On the rear side of this, suspended from a rod _3_ carried in a
bracket _4_, were a number of carbon rods or pendants _5_, loosely
resting against a rod _2_, carried on a bracket _6_ also mounted on
the rear of the diaphragm. The pivotal rod _3_ and the rod _2_,
against which the pendants rested, were sometimes, like the pendant
rods, made of carbon and sometimes of metal, such as brass. When the
diaphragm vibrated, the intimacy of contact between the pendant rod
_5_ and the rod _2_ was altered, and thus the resistance of the path
through all of the pendant rods in multiple was changed.
[Illustration: Fig. 39. Multiple-Electrode Transmitter]
A multitude of forms of such transmitters came into use in the early
eighties, and while they in some measure remedied the difficulty
encountered with the Blake transmitter, _i.e._, of not being able to
carry a sufficiently large current, they were all subject to the
effects of extreme sensitiveness, and would rattle or break when
called upon to transmit sounds of more than ordinary loudness.
Furthermore, the presence of such large masses of material, which it
was necessary to throw into vibration by the sound waves, was
distinctly against this form of transmitter. The inertia of the moving
parts was so great that clearness of articulation was interfered with.
Granular Carbon. The idea of employing a mass of granular carbon,
supported between two electrodes, one of which vibrated with the sound
waves and the other was stationary, was proposed by Henry Hunnings in
the early eighties. While this idea forms the basis of all modern
telephone transmitters, yet it did not prevent the almost universal
adoption of the single-contact form of instrument during the next
decade.
Western Electric Solid-Back Transmitter. In the early nineties,
however, the granular-carbon transmitter came into its own with the
advent and wide adoption of the transmitter designed by Anthony C.
White, known as the _White_, or _solid-back_, transmitter. This has
for many years been the standard instrument of the Bell companies
operating throughout the United States, and has found large use
abroad. A horizontal cross-section of this instrument is shown in Fig.
40, and a rear view of the working parts in Fig. 41. The working parts
are all mounted on the front casting _1_. This is supported in a cup
_2_, in turn supported on the lug _3_, which is pivoted on the
transmitter arm or other support. The front and rear electrodes of
this instrument are formed of thin carbon disks shown in solid black.
The rear electrode, the larger one of these disks, is securely
attached by solder to the face of a brass disk having a rearwardly
projecting screw-threaded shank, which serves to hold it and the rear
electrode in place in the bottom of a heavy brass cup _4_. The front
electrode is mounted on the rear face of a stud. Clamped against the
head of this stud, by a screw-threaded clamping ring _7_, is a mica
washer, or disk _6_. The center portion of this mica washer is
therefore rigid with respect to the front electrode and partakes of
its movements. The outer edge of this mica washer is similarly clamped
against the front edge of the cup _4_, a screw-threaded ring _9_
serving to hold the edge of the mica rigidly against the front of the
cup. The outer edge of this washer is, therefore, rigid with respect
to the rear electrode, which is fixed. Whatever relative movement
there is between the two electrodes must, therefore, be permitted by
the flexing of the mica washer. This mica washer not only serves to
maintain the electrodes in their normal relative positions, but also
serves to close the chamber which contains the electrodes, and,
therefore, to prevent the granular carbon, with which the space
between the electrodes is filled, from falling out.
[Illustration: Fig. 40. White Solid-Back Transmitter]