He will thus be enabled to select the instrument best suited
for the particular purpose he may have in view.
The first real improvement in this direction, as in so many others, is due
to the genius of Sir William Siemens. His first attempt was a calorimetric
pyrometer, in which a mass of copper at the temperature required to be
known is thrown into the water of a calorimeter, and the heat it has
absorbed thus determined. This method, however, is not very reliable, and
was superseded by his well-known electric pyrometer. This rests on the
principle that the electric resistance of metal conductors increases with
the temperature. In the case of platinum, the metal chosen for the
purpose, this increase up to 1,500 deg.C. is very nearly in the exact
proportion of the rise of temperature. The principle is applied in the
following manner: A cylinder of fireclay slides in a metal tube, and has
two platinum wires one one-hundredth of an inch in diameter wound round it
in separate grooves. Their ends are connected at the top to two
conductors, which pass down inside the tube and end in a fireclay plug at
the bottom. The other ends of the wires are connected with a small
platinum coil, which is kept at a constant resistance.
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