ntrol, it becomes
apparent that pyrometers could not be interchanged between furnaces
near and far from the instrument without affecting the uniformity
of product from each furnace.

Calibration can best be done without disturbing the working pyrometer,
by inserting the master instrument into each furnace separately, place
it alongside the hot junction of the working pyrometer, and compare
the reading given on the indicator connected with the multiple
switch.

Protection tubes should be replaced when cracked, as it is important
that no foreign substance is allowed to freeze in the tube, so
that the enclosed junction becomes a part of a solid mass joined
in electrical contact with the outside protecting tube. Wires over
the furnaces must be carefully inspected from time to time, as no
true reading can be had on an instrument, if insulation is burned
off and short circuits result.

If the standard calibrating instrument used contains a dry battery,
it should be examined from time to time to be sure it is in good
condition.

THE LEEDS AND NORTHRUP POTENTIOMETER SYSTEM

The potentiometer pyrometer system is both flexible and substantial
in that it is not affected by the jar and vibration of the factory
or the forge shop. Large or small couples, long or short leads
can be used without adjustment. The recording instrument may be
placed where it is most convenient, without regard to the distance
from the furnace.

ITS FUNDAMENTAL PRINCIPLE.--The potentiometer is the electrical
equivalent of the chemical balance, or balance arm scales. Measurements
are made with balance scales by varying known weights until they
equal the unknown weight. When the two are equal the scales stand
at zero, that is, in the position which they occupy when there is
no weight on either pan; the scales are then said to be balanced.
Measurements are made with the potentiometer by varying a known
electromotive force until it equals the unknown; when the two are
equal the index of the potentiometer, the galvanometer needle,
stands motionless as it is alternately connected and disconnected.
The variable known weights are units separate from the scales, but
the potentiometer provides its own variable known electromotive
force.

The potentiometer provides, first, a means of securing a known
variable electromotive force and, second, suitable electrical
connections for bringing that electromotive force to a point where
it may be balanced against the