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We have finally to determine the position and magnitude of the invisible radiation which produces these results. For this purpose we employ a particular form of the thermo-pile. Its face is a rectangle, which by movable side-pieces can be rendered as narrow as desirable.
This instrument, which is called a thermo-electric pile, or more briefly a thermo-pile, consists of thin bars of bismuth and antimony, soldered alternately together at their ends, but separated from each other elsewhere.
From the ends of this 'thermo-pile' wires pass to a galvanometer, which consists of a coil of covered wire, within and above which are suspended two magnetic needles, joined to a rigid system, and carefully defended from currents of air.
Introducing our ray-filter, the thermo-pile, playing the part of an eye as regards the invisible radiation, receives no heat when the eye receives no light; but when the mica is so turned as to make its planes of vibration oblique to those of the polarizer and analyzer, the heat immediately passes through.
The action of the thermo-pile, like that of a voltaic cell, can be reversed. By sending a current through the couple from the antimony to the bismuth we shall find the junction cooled. This "Peltier effect," as it is termed, after its discoverer, has been known to freeze water, but no practical application has been made of it.
When heat is the antecedent of electricity, as in the thermo-pile, that which is turned into the pile we know to be molecular motion of a definite kind. That which comes out of it must be some equivalent motion, and if all that went in were transformed, then all that came out would be transformed, call it by what name we will and let its amount be what it may.
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