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168 Fbrging-Stamping - Heat Treating
near a heat treating furnace, or may result merely
from variations in atmospheric temperature. The resistance
of the couple itself will of course change when
it is placed in a furnace.
In order to prevent fluctuations in line resistance
from having too great an effect upon the readings of
the instrument, modern millivoltmeters have in them,
as part of their circuit, a spool of resistance wire of
rather high resistance, generally about 500 ohms.
(Visible in Fig. 94.) This is made of an alloy (raan-
/lajusting Screw for Jewel
Brass Yoka
Spiral Spring
Carri es Current
to Coil
Movable coil
Pole Piece
Soft Iron
Kig.92.
Adjusting Scre^j
for Spring
Insulated from frame
Carries Current to Spring
Pointer
Counterweight
—itationory
Soil Iron Core
Scale, Showing
Deflection of
Coil.
Calibrated in
Millivolts or
Degrees.
Permanent
Magnet
Steel
Pole Piece.
Soft Iron
ganin) whose resistance changes very little at ordinary
atmospheric temperatures. The resistance of the couple,
the leads, and the other parts of the circuit is small
in comparison with the resistance of this coil. Changes
in their resistance, therefore, produce only small
changes in the total resistance of the circuit, and therefore
produce correspondingly small errors in the readings
of the instrument. The complete electrical circuit
of a millivoltmeter with resistance coil, and the
thermocouple and leads, is illustrated in Fig. 95.
May, 1925
The introduction of a high resistance reduces the
amount of current which the e.m.f. of the thermocouple
will produce in the circuit, and therefore makes it
necessary to use more delicate springs to control the
moving coil, and to reduce the friction of the pivot and
jewel bearings to a very low value. A certain sacrifice
in the ruggedness of the instrument is therefore
entailed. This is counterbalanced to some extent by
using a greater number of turns in the movable coil,
and increasing the strength of the permanent magnet.
Cold Junction Compensation At Instrument.
The preferred practice in most modern heat treating
plants is to bring the cold junction to the instrument
by means of extension leads, and to compensate
for the cold junction temperature either by a manual
adjustment or by some automatic device in the instrument.
Where high accuracy is not required, it is sufficient
to adjust the pointer of the millivoltmeter, so
that, with no current flowing, i.e., the thermocouple
disconnected, it will indicate room temperature (instead
of zero temperature). This is done by moving
the zero adjustment of the instrument. When the
FIG. 93—Movable coil, pointer and springs.
couple is connected, the reading of the pointer will
Brass Yoke
to Support Core
Lines of Maanetic
Force or "Flux"
then correspond to the deflection caused by the effective
e.m.f. (hot junction minus cold junction) plus
the zero setting, and therefore will indicate very nearly
the true hot junction temperature. There is a
Fig. 92 b
slight error in this method because the temperaturee.m.f.
scale for base metal couples is not strictly uni­
FIG. 92—(a) Diagram of movable coil mounted in pivots and form over the working range. The error is greater
jewels, (b) Simplified view of millivoltmeter, showing pole for noble metal couples than for base metal couples
pieces and core.
and should not be used with the former. There is also
the disadvantage that the operator may forget or neglect
to change the zero setting when the temperature
of the room changes. If the cold junction temperature
is set as 75 deg. and the temperature of the room rises
to 100 deg. F. the pyrometer will indicate about 25
deg. too low; if the temperature falls to 50 deg. F. the
reading will be about 25 deg. too high.
Diverse methods of automatically compensating for
variations in cold junction temperature have been developed
by pyrometer manufacturers. One method
(Patented), consists in attaching a thermostatic or