_Example._—A thermocouple and leads have a resistance of 5 ohms and are subject to alterations amounting to 1 ohm. To find the errors resulting when indicators of resistances 800, 400, and 50 ohms respectively are used. 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 (D_{1}) is noted, and resistances (R) are then unplugged until the deflection (D_{2}) is approximately one-half of D_{1}. The resistance (G) of the indicator, ignoring that of the wires, is then given by the formula D_{2}R G = ─────────────── (D_{1} - D_{2}) 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. [Illustration: FIG. 14.—INDICATOR WITH EDGEWISE SCALE.]