The Leeds And Northrup Potentiometer System
Category: PYROMETRY AND PYROMETERS
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
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 unknown electromotive force of the
couple. The two are connected with opposite polarity, or so that
the two e.m.f.s oppose one another. So long as one is stronger
than the other a current will flow through the couple; when the
two are equal no current will flow.
Figure 107 shows the wiring of the potentiometer in its simplest
form. The thermo-couple is at H, with its polarity as shown by
the symbols + and -. It is connected with the main circuit of the
potentiometer at the fixed point D and the point G.
A current from the dry cell Ba is constantly flowing through the
main, or so-called potentiometer circuit, ABCDGEF. The section
DGE of this circuit is a slide wire, uniform in resistance throughout
its length. The scale is fixed on this slide wire. The current
from the cell Ba as it flows through DGE, undergoes a fall
in potential, setting up a difference in voltage, that is, an
electromotive force, between D and E. There will also be
electromotive force between D and all other points on the slide
wire. The polarity of this is in opposition to the polarity of the
thermo-couple which connects into the potentiometer at D and
at G. By moving G along the slide wire a point is found where
the voltage between D and G in the slide wire is just equal to
the voltage between D and G generated by the thermo-couple. A
galvanometer in the thermo-couple circuit indicates when the balance
point is reached, since at this point the galvanometer needle will
stand motionless when its circuit is opened and closed.
The voltage in the slide wire will vary with the current flowing
through it from the cell Ba and a means of standardizing this is
provided. SC, Fig. 111, is a cadmium cell whose voltage is constant.
It is connected at two points C and D to the potentiometer
circuit whenever the potentiometer current is to be standardized.
At this time the galvanometer is thrown in series with SC. The
variable rheostat R is then adjusted until the current flowing
is such that as it flows through the standard resistance CD,
the fall in potential between C and D is just equal to the
voltage of the standard cell SC. At this time the galvanometer
will indicate a balance in the same way as when it was used with
a thermo-couple. By this operation the current in the slide wire
DGE has been standardized.
DEVELOPMENT OF THE WIRING SCHEME OF THE COLD-END COMPENSATOR.--The
net voltage generated by a thermo-couple depends upon the temperature
of the hot end and the temperature of the cold end. Therefore, any
method adopted for reading temperature by means of thermo-couples
must in some way provide a means of correcting for the temperature of
the cold end. The potentiometer may have either of two very simple
devices for this purpose. In one form the operator is required
to set a small index to a point on 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, in which the assumption is
made that the reader already understands the potentiometer principle
as described above.
As previously explained the voltage of the thermo-couple is measured
by balancing it against the voltage drop DG in the potentiometer.
As shown in Fig. 111, the magnitude of the balancing voltage is
controlled by the position of G. Make D movable as shown in Fig.
112 and the magnitude of the voltage DG may be varied either from
the point D or the point G. This gives a means of compensating
for cold end changes by setting the slider D. As the cold end
temperature rises the net voltage generated by the couple decreases,
assuming the hot end temperature to be constant. To balance this
decreased voltage the slider D is moved along its scale to a new
point nearer G. In other words, the slider D is moved along
its scale until it corresponds to the known temperature of the cold
end and then the potentiometer is balanced by moving the slider
G. The readings of G will then be direct.
The same results will be obtained if a slide wire upon which D
bears is in parallel with the slide wire of G, as shown in Fig.
AUTOMATIC COMPENSATOR.--It should be noted that the effect of moving
the contact D, Fig. 113, is to vary the ratio of the resistances
on the two sides of the point D in the secondary slide wire. In
the recording pyrometers, an automatic compensator is employed.
This automatic compensator varies the ratio on the two sides of
the point D in the following manner:
The point D, Fig. 114, is mechanically fixed; on one side of
D is the constant resistance coil M, on the other the nickel
coil N. N is placed at or near the cold end of the thermo-couple
(or couples). Nickel has a high temperature coefficient and the
electrical proportions of M and N are such that the resistance
change of N, as it varies with the temperature of the cold end,
has the same effect upon the balancing voltage between D and
G that the movement of the point D, Fig. 114, has in the
Instruments embodying these principles are shown in Figs. 115 to
117. The captions making their uses clear.
Next: Placing The Thermo-couples
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