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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 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
press- ure 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 could not be made
at that time, because the tasimeter adjustment
was too delicate, and at the best the
galvanometer deflections were so marked that they
could not be kept within the limits of the
scale. The sensitiveness of the instrument may
be easily comprehended when it is stated that the
heat of the hand thirty feet away from the
cone-like funnel of the tasimeter will so affect
the galvanometer as to cause the spot of light to
leave the scale.
This instrument can also be used to indicate
minute changes of moisture in the air by
substituting a strip of gelatine in place of the
vulcanite. When so arranged a moistened piece
of paper held several feet away will cause a
minute expansion of the gelatine strip, which
effects a pressure on the carbon, and causes a
variation in the circuit sufficient to throw the
spot of light from the galvanometer mirror off
the scale.
The tasimeter has been used to demonstrate heat
from remote stars (suns), such as Arcturus.
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