a slot in a metal plate, C, which in turn rests upon a carbon button.
This latter and the metal plate are connected in an electric circuit
which includes a battery and a sensitive galvanometer. A vulcanite or
other strip is easily affected by differences of temperature,
expanding and contracting by reason of the minutest changes. Thus, an
infinitesimal variation in its length through expansion or contraction
changes the pressure on the carbon and affects the resistance of the
circuit to a corresponding degree, thereby causing a deflection of
the galvanometer; a movement of the needle in one direction denoting
expansion, and in the other contraction. The strip, A, is first put
under a slight pressure, deflecting the needle a few degrees from zero.
Any subsequent expansion or contraction of the strip may readily
be noted by further movements of the needle. In practice, and for
measurements of a very delicate nature, the tasimeter is inserted in one
arm of a Wheatstone bridge, as shown at A in the diagram (Fig. 2). The
galvanometer is shown at B in the bridge wire, and at C, D, and E there
are shown the resistances in the other arms of the bridge, which are
adjusted to equal the resistance of the tasimeter circuit. The battery
is shown at F. This arrangement tends to obviate any misleading
deflections that might arise through changes in the battery.

The dial on the front of the instrument is intended to indicate the
exact amount of physical expansion or contraction of the strip. This is
ascertained by means of a micrometer screw, S, which moves a needle, T,
in front of the dial. This screw engages with a second and similar screw
which is so arranged as to move the strip of vulcanite up or down. After
a galvanometer deflection has been obtained through the expansion or
contraction of the strip by reason of a change of temperature, a similar
deflection is obtained mechanically by turning the screw, S, one way or
the other. This causes the vulcanite strip to press more or less
upon the carbon button, and thus produces the desired change in the
resistance of the circuit. When the galvanometer shows the desired
deflection, the needle, T, will indicate upon the dial, in decimal
fractions of an inch, the exact distance through which the strip has
been moved.

With such an instrument as the above, Edison demonstrated the existence
of heat in the corona at the above-mentioned total eclipse of the sun,
but exact determinations co