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experiments is a plate of copper, against the back of which
a steady sheet of flame is permitted to play. On emerging
from the copper, the waves, in the first instance, pass
through a space devoid of air, and then enter a hollow
glass cylinder, three feet long and three inches wide. The
two ends of this cylinder are stopped by two plates of rock-
salt, this being the only solid substance which offers a
scarcely sensible obstacle to the passage of the calorific
waves. After passing through the tube, the radiant heat
falls upon the anterior face of a thermo-electric pile,1 which
instantly applies the heat to the generation of an electric
current. This current conducted round a magnetic needle
deflects it, and the magnitude of the deflection is a measure
of the heat falling upon the pile. This famous instrument,
and not an ordinary thermometer, is what we shall use in
these inquiries, but we shall use it in a somewhat novel
way. As long as the two opposite faces of the thermo-
electric pile are kept at the same temperature, no matter
how high that may be, there is no current generated. The
current is a consequence of the déference of temperature
between the two Opposite faces of the pile. Hence, if after
the anterior face has received the heat from our radiating
source, a second source, which we may call the compensat-
ing source, be permitted to radiate against the posterior
face, this latter radiation'will tend to neutralize the former.
When the neutralization is perfect, the magnetic needle
connected with the pile is no longer deflected, but points to
the zero of the graduated circle over which it hangs.

And now let us suppose the glass tube, through which
pass the waves from the heated plate of copper, to be ex-
hausted by an air-pump, the two sources of heat acting at
the same time on the two opposite faces of the pile. Per-
fectly equal quantities of heat being imparted to the two

1 In the Appendix to the first chapter of “Heat as a Mode of Motion,”
the construction of the thermo-electric pile is fully explained.

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