[Footnote 1: Probably most of our readers know that the
thermopile consists of a number of little bars of two different
metals, connected in pairs, and having the ends joined in a
conducting circuit with a galvanometer. If, now, one set of the
junctures is heated more than the other set, a current of
electricity will be generated, which will affect the
galvanometer. The bars are usually made of bismuth and antimony
though iron and German silver answer pretty well. They are
commonly about half or three-quarters of an inch long, and about
half as large as an ordinary match. The "pile" is made of from
fifty to a hundred such bars packed closely, but insulated by
thin strips of mica, except just at the soldered junctions. With
an instrument of this kind and a very delicate galvanometer,
Professor Henry found that the heat from a person's face could be
perceived at a distance of several hundred feet. There is
however, some doubt whether he was not mistaken in respect to
this extreme sensitiveness.
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