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