It has been pointed out that with accurate measuring devices, a resistance corresponding to a change of 1° C. can be measured; and it might appear at first sight that the resistance method is considerably more accurate in practice than the thermo-electric. If a perfectly constant temperature were to be measured, a resistance pyrometer would undoubtedly give a closer indication; but constancy to 10° C. is seldom possible with gas-fired or coal furnaces or other hot spaces in which pyrometers are used. The accuracy of a pyrometer under workshop conditions therefore depends upon the rapidity with which it responds to temperature fluctuations, which condition will evidently be influenced by the thermal conductivity of the sheath. As it is necessary to protect a resistance pyrometer with a porcelain or silica sheath, both of which are poor conductors of heat, this instrument is in consequence not capable of following a rapidly changing temperature. The same applies to the magnesia packing used in the Siemens form; whereas a thermo-electric pyrometer is often sufficiently shielded by an iron tube, which transmits heat with a fair degree of freedom. The superior delicacy of the resistance method is therefore nullified by the sluggishness of its indications; and for reading changing temperatures the thermo-electric pyrometer is at least equally accurate. If, however, a constant temperature can be obtained, as in the determination of melting points, or when using experimental furnaces capable of exact regulation, the steady temperature reading may be secured with greater precision by using the resistance pyrometer. =Indicators for Resistance Pyrometers.=—All existing indicators for resistance pyrometers are in reality outfits for measuring resistance, either by the Wheatstone bridge, differential galvanometer, or other method, the resistance being translated on the dial into corresponding temperatures. Typical examples will now be described. [Illustration: FIG. 35.—SIEMENS’ DIAL INDICATOR.]