the temperature, allowance is made on the
brass sliding scales, which are divided according to the metal used
for the pyrometer cylinder _d_. It will therefore be understood that
a different sliding scale is required for the particular kind of
metal of which a cylinder is composed. In order to obtain accurate
measurements, each sliding scale must be used only in conjunction
with its own thermometer, and in case the latter breaks a new scale
must be made and graduated for the new thermometer.

The water pyrometer is used as follows:

Exactly one pint (0.568 liter) of clean water, perfectly distilled
or rain water, is poured into the copper vessel, and the pyrometer
is left for a few minutes to allow the thermometer to attain the
temperature of the water.

The brass scale _c_ is then set with its pointer opposite the
temperature of the water as shown by the thermometer. Meanwhile
one of the metal cylinders has been exposed to the high temperature
which is to be measured, and after allowing sufficient time for
it to acquire that temperature, it is rapidly removed and dropped
into the pyrometer vessel without splashing any of the water out.

The temperature of the water will rise until, after a little while,
the mercury of the thermometer has become stationary. When this
is observed the degrees of the thermometer are read off, as well
as those on the brass scale _c_ opposite the top of the mercury.
The sum of these two values together gives the temperature of the
flue, furnace or other heated space in which the metal cylinder
had been placed. With cylinders of copper and iron, temperatures up
to 1,800 deg.F. (1,000 deg.C.) can be measured, but with platinum cylinders
the limit is 2,700 deg.F. (1,500 deg.C.).

For ordinary furnace work either copper or wrought-iron cylinders
may be used. Iron cylinders possess a higher melting point and have
less tendency to scale than those of copper, but the latter are
much less affected by the corrosive action of the furnace gases;
platinum is, of course, not subject to any of these disadvantages.

The weight to which the different metal cylinders are adjusted is
as follows:

Copper 137.0 grams
Wrought-iron 112.0 grams
Platinum 402.6 grams

In course of time the cylinders lose weight by scaling; but tables
are provided giving multipliers for the diminished weights, by
which the reading on the brass scale should be multiplied.

THE THERMO-COUPLE

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