BBBBflt] «BlJIUrIrlr - Clpdigital.org

17i)
nometer is then in -eric- with S.C. The variable rheostat
R is then adjusted until the current flowing is
such that as it flows through the slide wire DGE and
the standard resistance EF, the fall in potential between
1) and F is just equal to the voltage of the
standard cell S. C At this time the galvanometer will
indicate a balance in the same way as when it was
used with a thermocouple. By this operation the current
in the slide wire DGE has been standardized.
The standard cell is then disconnected and the thermocouple
connected by closing the contact shown between
Galv. and the + side of II.
Automatic Cold Junction Compensator.
The net electromotive force generated by a thermocouple
depends upon the temperature of the hot junction
and the temperature of the cold junction. An)
method adopted for reading temperatures by means of
thermocouples must in some way provide a means oi
correcting for the temperature of the cold end- For
this purpose, the potentiometer may have either of two
very simple devices. In one form the operator is
required to set a small index to a point mi a scale corresponding
to the known cold junction temperature.
In the other form, an even more simple automatic compensator
is employed. The principle of each is described
in the succeeding paragraphs.
As previously explained, the electromotive force of
the thermocouple is measured by balancing it against
the potential DG in the slide wire of the potentiometer.
As shown in Fig. 96-b the magnitude of the balancing
potential is controlled by the position of G. Make D
movable as shown in Fig. 96-c and the magnitude of
the potential DG may be varied either from the point
D or the point G. This gives a means for compensating
for cold junction changes by setting the slider D.
As the cold junction temperature rises, the net electromotive
force generated by the thermocouple decreases,
assuming the hot junction temperature to be constant.
To balance this decreased electromotive force the
slider 1) is moved along its scale to a new point nearer
G. In other words, the slider I) is moved along its
scale until it corresponds to the known temperature of
the cold junction, and then the potentiometer is balanced
by moving the slider G. The readings of G will
then give the temperature of the hot junction directly.
The same results will be obtained if another slide
wire upon which D bears is placed in parallel with the
slide wire of G. as shown in Fig. 96-d. It is such a
slide wide, with a temperature scale fixed upon it, that
forms the manually operated cold junction compensator.
It should be noted that the effect of moving the
contact D, Fig. 96-d. is either to increase N and decrease
F. or to decrease X and increase M ; in other
words, the ratio between X and M is varied. In Leeds
oc Xorthrup indicators and recorders employing the
automatic cold junction compensator, the ratio of X
to M is varied automatically, in the following manner:
The point D. Fig. 96-e. is mechanically fixed; on
one side of D is the constant resistance coil M, on the
other the nickel coil X. X is placed at or near the cold
junction of the thermocouple. Nickel has a high temperature
coefficient (that is. its resistance changes
about .25 per cent for every degree F.) and the electrical
proportions of M and N are such that the resistance
change of X, as it varies with the temperature of
the cold junction, has practically the same effect upon
Forging - Stamping - Heat Treating
May. 1925
the balancing potential between I) and G that the
movement of the point D has in the hand-operated
compensator of Fig. 96-d.
Fig. 96-f shows a modification of Fig. 96-e to meet
the condition where the cold junction of the thermocouples
is to be located at some distance from the measuring
instrument."
WWViA/V-
"0
E F
FIG. 96—Diagram, circuit potentiometer pyrometer.
The potentiometer is the most accurate means of
measuring the e.m.f. of a thermocouple. Since no current
flows in the thermocouple circuit when the reading
is taken, it is independent of the resistance of the
line, and is therefore unaffected by changes in temperature,
or length of the lead wires. If the resistance
of the lead wires is very high, as might result from a
bad connection, or excessive length, the deflections of
the galvanometer will be weak, and it will be difficult
to locate the point of true balance on the slide wire.
The galvanometer is used only to indicate when the
current is zero, and therefore variations in the restor-