y calibrated
for a nominal charge, and returned with the voltmeter readings
of a series of temperatures covering practically the whole range
of the couple. This couple is then used only for checking those
in daily use.

Pyrometer couples are more or less expensive, and should be cared
far when in use. The wires of the couple should be insulated from
each other by fireclay leads or tubes, and it is well to encase them
in a fireclay, porcelain, or quartz tube to keep out the furnace
gases, which in time destroy the hot junction. This tube of fireclay,
or porcelain, etc., should be protected against breakage by an
iron or nichrome tube, plugged or welded at the hot end. These
simple precautions will prolong the life of a couple and maintain
its precision longer.

Sometimes erroneous temperatures are recorded because the "cold
end" of the couple is too near the furnace and gets hot. This always
causes a temperature reading lower than the actual, and should be
guarded against. It is well to keep the cold end cool with water,
a wet cloth, or by placing it where coal air will circulate around
it. Best of all, is to have the cold junction in a box, together
with a thermometer, so that its temperature may definitely be known.
If this temperature should rise 20 deg.F. on a hot day, a correction of
20 deg.F. should be added to the pyrometer reading, and so on. In the
most up-to-date installations, this cold junction compensation is
taken care of automatically, a fact which indicates its importance.

Optical pyrometers are often used where it is impracticable to
use the thermo-couple, either because the temperature is so high
that it would destroy the couple, or the heat to be measured is
inaccessible to the couple of ordinary length. The temperatures of
slag or metal in furnaces or running through tap-holes or troughs
are often measured with optical pyrometers.

In one type of optical pyrometer, the observer focuses it on the
metal or slag and moves an adjustable dial or gage so as to get
an exact comparison between the color of the heat measured with
the calor of a lamp or screen in the pyrometer itself. This, of
course, requires practice, and judgment, and brings in the personal
equation. With care, however, very reliable temperature measurements
may be made. The temperatures of rails, as they leave the finishing
pass of a rolling mill, are measured in this way.

Another type of optical pyrometer is focused on the body, t