From Ohm’s law, C = E/R, the variation in C, with E constant, will be 1 in 805, 1 in 405, and 1 in 55 respectively. As the indications are proportional to the current, the alterations caused will be approximately ⅛ per cent., ¼ per cent., and 2 per cent. The first two may be ignored; the last may be quite serious and lead to the failure of an operation.
It will be seen from the foregoing that low-resistance indicators should only be used for fixed thermocouples and short leads not subject to temperature changes, or, in other words, in a circuit of fixed resistance.
The resistance of an indicator, when unknown, may be found by the following method, suggested by the author:—A resistance box is joined at one end to one terminal of the indicator. To the other terminal a fairly stout iron wire, 18 inches long, is connected, and a similar length of constantan wire is coupled to the other end of the resistance box. The free ends of the wires are twisted into a junction which is dipped into boiling water. The deflection obtained with no resistance in the box (D1) is noted, and resistances (R) are then unplugged until the deflection (D2) is approximately one-half of D1. The resistance (G) of the indicator, ignoring that of the wires, is then given by the formula
| D2R | ||
| G | = | —————— |
| D1 - D2 |
as may readily be proved from Ohm’s law, E being constant. This method is extremely simple and reasonably accurate.
Reliable indicators are now procurable from many instrument-makers at a comparatively small cost, progress in this direction having been most marked in recent years, particularly in the case of pivoted instruments. The most convenient form for workshop use is made with an edgewise scale ([fig. 14]) and may be placed in a suitable position fixed to a bracket. The flat-scale pattern is preferable for use on a laboratory table, or for a portable pyrometer. The sector pattern is also good for workshop use, the dial being visible from a distance.
Fig. 14.—Indicator with Edgewise Scale
Standardizing of Indicators to read Temperatures directly.—The temperature scale of an indicator, for use with a given thermal couple, is always marked by the maker in the case of instruments furnished with a pointer, and, generally speaking, is correct within reasonable limits. It is customary and necessary to send with the instrument a statement of the cold-junction temperature for which the markings are correct; say 20° C. or 60° F. The user should then endeavour to maintain the cold junction at this specified temperature when taking a reading, or otherwise a considerable error may be introduced. It is highly desirable, however, that the user should be able to perform the standardizing himself, if only for checking purposes; and when using a mirror galvanometer as indicator it is necessary to standardize on the spot at which the instrument is fixed. Ability to prepare a temperature scale is further useful, inasmuch as any good millivoltmeter, of range 0 to 20 millivolts, may be used for thermo-electric work of all kinds, and may be calibrated for different junctions, a suitable series resistance being added to enable E.M.F.’s higher than 20 millivolts to be measured. Such an instrument may thus be made extremely useful, both in the workshop and laboratory.