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country, being superseded by the various forms of granular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.