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 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.
FIG. 110.—Simple potentiometer.
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.
FIG. 111.—Standard cell potentiometer.
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.
FIG. 112.—Hand adjusted cold-end compensator.
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.
FIG. 113.—Another type of compensator.
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.
FIG. 114.—Automatic cold-end compensator.