ts. The current is very
small, about 1/10 amp., as it is only necessary to operate the relay
which in turn operates the switch or valve. A small motor is used to
depress the pointer at regular intervals. The contact-making device
is adjustable throughout the scale range of the instrument, and an
index pointer indicates the point on the instrument at which the
temperature is being controlled. The space between the two contacts
on the high and low side, separated by insulating material, is
equivalent to 1 per cent of the scale range. A control of temperature
is therefore possible within 1 per cent of the total scale range.
Figure 131 shows this attached to a small furnace.

[Illustration: FIG. 132.--Portable thermocouple testing molten brass.]

PYROMETERS FOR MOLTEN METAL

Pyrometers for molten metal are connected to portable thermocouples
as in Fig. 132. Usually the pyrometer is portable, as shown in
this case, which is a Brown. Other methods of mounting for this
kind of work arc shown in Figs. 133 and 134. The bent mountings
are designed for molten metal, such as brass or copper and are
supplied with either clay, graphite or carborundum tubes. Fifteen
feet of connecting wire is usually supplied.

The angle mountings, Fig. 134, are recommended for baths such as
lead or cyanide. The horizontal arm is usually about 14 in. long,
and the whole mounting is easily taken apart making replacements
very easy. Details of the thermo-couple shown in Fig. 132 are given
in Fig. 135. This is a straight rod with a protector for the hand
of the operator. The lag in such couples is less than one minute.
These are Englehard mountings.

PROTECTORS FOR THERMO-COUPLES

Thermo-couples must be protected from the danger of mechanical
injury. For this purpose tubes of various refractory materials
are made to act as protectors. These in turn are usually protected
by outside metal tubes. Pure wrought iron is largely used for this
purpose as it scales and oxidizes very slowly. These tubes are
usually made from 2 to 4 in. shorter than the inner tubes. In lead
baths the iron tubes often have one end welded closed and are used
in connection with an angle form of mounting.

[Illustration: FIG. 133.--Bent handle thermocouple with protector.]

Where it is necessary for protecting tubes to project a considerable
distance into the furnace a tube made of nichrome is frequently used.
This is a comparatively new alloy which stands high temperatures