h the application of the thermo-couple, the measurement of
temperatures, between, say, 700 and 2,500 deg.F., was made more simple
and precise. The theory of the thermo-couple is simple; it is that
if two bars, rods, or wires of different metals are joined together
at their ends, when heated so that one junction is hotter than the
other, an electromotive force is set up through the metals, which
will increase with the increase of the _difference_ of temperature
between the two junctions. This electromotive force, or voltage, may
be measured, and, from a chart previously prepared, the temperature
determined. In most pyrometers, of course, the temperatures are
inscribed directly on the voltmeter, but the fact remains that
it is the voltage of a small electric current, and not heat, that
is actually measured.

There are two common types of thermo-couples, the first making use
of common, inexpensive metals, such as iron wire and nichrome wire.
This is the so-called "base metal" couple. The other is composed of
expensive metals such as platinum wire, and a wire of an alloy of
platinum with 10 per cent of rhodium or iridium. This is called
the "rare metal" couple, and because its component metals are less
affected by heat, it lasts longer, and varies less than the base
metal couple.

The cold junction of a thermo-couple may be connected by means
of copper wires to the voltmeter, although in some installations
of base metal couples, the wires forming the couple are themselves
extended to the voltmeter, making copper connections unnecessary.
From the foregoing, it may be seen that accurately to measure the
temperature of the hot end of a thermo-couple, we _must know the
temperature of the cold end_, as it is the _difference_ in the
temperatures that determines the voltmeter readings. This is absolutely
essential for precision, and its importance cannot be over-emphasized.

When pyrometers are used in daily operation, they should be checked
or calibrated two or three times a month, or even every week. Where
there are many in use, it is good practice to have a master pyrometer
of a rare metal couple, which is used only for checking up the
others. The master pyrometer, after calibrating against the melting
points of various substances, will have a calibration chart which
should be used in the checking operation.

It is customary now to send a rare metal couple to the Bureau of
Standards at Washington, where it is very carefull