The 'micro-tasimeter,' a delicate thermoscope, was constructed in 1878,
and is the outcome of Edison's experiments with the carbon button.
Knowing the latter to be extremely sensitive to minute changes of
pressure, for example, those of sonorous vibrations, he conceived the
idea of measuring radiant heat by causing it to elongate a thin bar or
strip of metal or vulcanite, bearing at one end on the button. To
indicate the effect, he included a galvanometer in the circuit of the
battery and the button. The apparatus consisted of a telephone button
placed between two discs of platinum and connected in circuit with the
battery and a sensitive galvanometer. The strip was supported so that
one end bore upon the button with a pressure which could be regulated by
an adjustable screw at the other. The strip expanded or contracted when
exposed to heat or cold, and thrust itself upon the button more or less,
thereby varying the electric current and deflecting the needle of the
galvanometer to one side or the other. The instrument was said to
indicate a change of temperature equivalent to one-millionth of a
degree Fahrenheit. It was tested by Edison on the sun's corona during
the eclipse observations of July 29, 1875, at Rawlings, in the
territory of Wyoming.
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