=Special Uses of Resistance Pyrometers.=—In all cases in which an exact reading is required, and a steady temperature can be secured, the resistance pyrometer can be used to advantage. Thus for accurate determinations of melting points and boiling points, or for exact readings of temperatures in experimental furnaces, a resistance pyrometer is superior to appliances of other kinds. On the other hand, it is not capable of responding to changes with the same rapidity as a thermal junction, and is therefore inferior for such purposes as the determination of recalescence points, or the temperature of exhaust gases from an internal combustion engine. The resistance method may be applied to atmospheric and very low temperatures (liquefied gases, etc.), to measure steady conditions with accuracy, nickel wire being sometimes used instead of platinum below 400° C. Many cold stores are fitted with resistance thermometers, the temperature being read directly on the galvanometer, which is placed across a Wheatstone bridge, and shows a deflection which depends upon the amount by which the bridge is thrown out of balance. Changes in the temperature of the resistance element may thus be read accurately. Whether the resistance method is suitable to a given purpose must be decided by the three factors: (1) temperature to be measured, which must not exceed 1000° C. continuously; (2) degree of accuracy required (a thermo-electric pyrometer giving results to 10° C.); (3) stability of the temperature measured, rapid changes not being readily shown by resistance pyrometers. One advantage of resistance pyrometers is that the readings are independent of the resistance of the wires used to connect the pyrometer with the indicator, as such wires are duplicated and opposed to each other in the measuring device, their resistance being thereby cancelled. Hence the same reading is obtained at any distance, and, in addition, the head of the pyrometer may vary in temperature to any extent without altering the reading. These are points of superiority over the thermo-electric method; but, on the other hand, resistance pyrometers and indicators are more costly, more fragile, more difficult to repair, require more skilled attention, and are more liable to get out of order when used for industrial purposes. These drawbacks have resulted in restricting the use of resistance pyrometers to special purposes, the general run of observations being conducted by means of thermo-electric pyrometers. CHAPTER V RADIATION PYROMETERS