The Leeds And Northrup Potentiometer System

: PYROMETRY AND PYROMETERS
: The Working Of Steel

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 electric l

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 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.

113.



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

hand-operated compensator.



Instruments embodying these principles are shown in Figs. 115 to

117. The captions making their uses clear.