he
temperature of which is to be measured. The rays converge in the
telescope on metal cells, heating them, and thereby generating a
small electric current, the voltage of which is read an a calibrated
voltmeter similar to that used with the thermo-couple. The best
precision is obtained when an optical pyrometer is used each time
under similar conditions of light and the same observer.

Where it is impracticable to use either thermo-couples or optical
pyrometers, "sentinels" may be used. There are small cones or cylinders
made of salts or other substances of known melting points and covering
a wide range of temperatures.

If six of these "sentinels," melting respectively at 1,300 deg., 1,350 deg.,
1,400 deg., 1,450 deg., 1,500 deg., and 1,550 deg.F., were placed in a row in a
furnace, together with a piece of steel to be treated, and the
whole heated up uniformly, the sentinels would melt one by one and
the observer, by watching them through an opening in the furnace,
could tell when his furnace is at say 1,500 deg. or between 1,500 deg. and
1,550 deg., and regulate the heat accordingly.

A very accurate type of pyrometer, but one not so commonly used as
those previously described, is the resistance pyrometer. In this
type, the temperature is determined by measuring the resistance to an
electric current of a wire which is at the heat to be measured. This
wire is usually of platinum, wound around a quartz tube, the whole
being placed in the furnace. When the wire is at the temperature of
the furnace, it is connected by wires with a Wheatstone Bridge, a
delicate device for measuring electrical resistance, and an electric
current is passed through the wire. This current is balanced by
switching in resistances in the Wheatstone Bridge, until a delicate
electrical device shows that no current is flowing. The resistance
of the platinum wire at the heat to be measured is thus determined
on the "Bridge," and the temperature read off on a calibration
chart, which shows the resistance at various temperatures.

These are the common methods used to-day for measuring temperatures,
but whatever method is used, the observer should bear in mind that
the greatest precision is obtained, and hence the highest efficiency,
by keeping the apparatus in good working order, making sure that
conditions are the same each time, and calibrating or checking
against a standard at regular intervals.

THE PYROMETER AND ITS USE

In the heat trea