the transmission of speech.... The carbon
transmitter was an experimental invention of a very high order of
merit.... Edison, by countless experiments, succeeded in advancing the
art. . . . That Edison did produce speech with solid electrodes before
Berliner is clearly proven.... The use of carbon in a transmitter is,
beyond controversy, the invention of Edison. Edison was the first to
make apparatus in which carbon was used as one of the electrodes....
The carbon transmitter displaced Bell's magnetic transmitter, and,
under several forms of construction, remains the only commercial
instrument.... The advance in the art was due to the carbon electrode of
Edison.... It is conceded that the Edison transmitter as apparatus is a
very important invention.... An immense amount of painstaking and highly
ingenious experiment preceded Edison's successful result. The discovery
of the availability of carbon was unquestionably invention, and it
resulted in the 'first practical success in the art.'"

VII. EDISON'S TASIMETER

THIS interesting and remarkable device is one of Edison's many
inventions not generally known to the public at large, chiefly because
the range of its application has been limited to the higher branches of
science. He never applied for a patent on the instrument, but dedicated
it to the public.

The device was primarily intended for use in detecting and measuring
infinitesimal degrees of temperature, however remote, and its conception
followed Edison's researches on the carbon telephone transmitter. Its
principle depends upon the variable resistance of carbon in accordance
with the degree of pressure to which it is subjected. By means of
this instrument, pressures that are otherwise inappreciable and
undiscoverable may be observed and indicated.

The detection of small variations of temperatures is brought about
through the changes which heat or cold will produce in a sensitive
material placed in contact with a carbon button, which is put in circuit
with a battery and delicate galvanometer. In the sketch (Fig. 1) there
is illustrated, partly in section, the form of tasimeter which Edison
took with him to Rawlins, Wyoming, in July, 1878, on the expedition to
observe the total eclipse of the sun.

The substance on whose expansion the working of the instrument depends
is a strip of some material extremely sensitive to heat, such as
vulcanite. shown at A, and firmly clamped at B. Its lower end fits into