equipment, the rapid displacement of open wires with cable, etc. Then, in August, 1877, Graham Bell showed the advantage of twisting 'the direct and return wires around one another, so they should be absolutely equidistant from the disturbing wires' in order to neutralize the effect of the inductive current and eliminate the noise.
Many experiments were made to invent an improved transmitter that would overcome the inductive effect and yet retain the marvelous simplicity of the hand telephone, with its entailed low cost of maintenance. But eventually it was perceived that the displacement of the magnets in the simple self-contained telephone was possible only
through the introduction of a battery current and the employment of much mechanism that has always carried relatively heavy maintenance charges.
In the winter of 1878-9, the more progressive companies began to install the Blake transmitter in combination with the rubber-encased Bell receiver and the magneto bell. At the close of 1878, 246 Blake transmitters were in service, and by July 1, 1879, the number had increased to 7,000. On noisy circuits this change afforded a marked improvement in service that was highly appreciated by local subscribers. Several modifications in the form of these telephone sets (Fig. 30) were sent out before a standard type was selected. Even then, as there were several factories licensed to manufacture under Bell patents, the output of each, while not essentially different, bore distinctive trade-marks. In each the battery wires were led into the
