Single Electrode. Blake.  The most notable example of the single-contact transmitter is the once familiar Blake instrument.  At one time this formed a part of the standard equipment of almost every telephone in the United States, and it was also largely used abroad.  Probably no transmitter has ever exceeded it in clearness of articulation, but it was decidedly deficient in power in comparison with the modern transmitters.  In this instrument, which is shown in Fig. 38, the variable-resistance contact was that between a carbon and a platinum electrode.  The diaphragm 1 was of sheet iron mounted, as usual in later transmitters, in a soft rubber gasket 2.  The whole diaphragm was mounted in a cast-iron ring 3, supported on the inside of the box containing the entire instrument.  The front electrode 4 was mounted on a light spring 5, the upper end of which was supported by a movable bar or lever 6, flexibly supported on a spring 7 secured to the casting which supported the diaphragm.  The tension of this spring 5 was such as to cause the platinum point to press lightly away from the center of the diaphragm.  The rear electrode was of carbon in the form of a small block 9, secured in a heavy brass button 10.  The entire rear electrode structure was supported on a heavier spring 11 carried on the same lever as the spring 5.  The tension of this latter spring was such as to press against the front electrode and, by its greater strength, press this against the center of the diaphragm.  The adjustment of the instrument was secured by means of the screw 12, carried in a lug extending rearwardly from the diaphragm supporting casting, this screw, by its position, determining the strength with which the rear electrode pressed against the front electrode and that against the diaphragm.  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The Blake transmitter has passed almost entirely out of use in this country, being superseded by the various forms of granular instruments, which, while much more powerful, are not perhaps capable of producing quite such clear and distinct articulation.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.