=Installations of Resistance Pyrometers.=—The resistance method cannot be so readily applied to the purpose of a centrally controlled installation as the thermo-electric, owing to the difficulty of producing a set of pyrometers exactly equal in resistance. The introduction of the ohmmeter method of measuring resistances, as in the Harris indicator (page 122), has, however, rendered this project feasible, as it is possible in this arrangement to bring a set of pyrometers to a common resistance by adding the requisite amount in the form of a wire of negligible temperature coefficient. Several instruments, brought thus to a zero resistance of 3 ohms, for example, may then be wired up to a Harris recorder, and will give closely identical results. For various reasons, however, a thermo-electric installation is preferable. =Management of Resistance Pyrometers.=—It is not advisable to use resistance pyrometers continuously above 900° C. (1650° F.), although an occasional reading may be taken up to 1200° C. (2190° F.). Great care must be taken that metallic vapours or furnace gases do not find access to the interior, and for this reason a cracked or defective sheath should immediately be replaced. As the resistance gradually changes, even when 900° C. is not exceeded, a reading should be checked at a fixed point in the neighbourhood of the working temperature, and allowance made for the observed error. Another method of correction recommended by some makers is to measure the resistance in ice, and to note how much this differs from the zero resistance noted when the indicator was marked, and to correct by simple proportion. Thus, if the observed resistance in ice were 10·2 ohms, the original having been 10·0 the reading on the indicator would be multiplied by 10·0/10·2 = 0·98, a correction which assumes a linear relation between resistance and temperature, and is therefore only approximate. Generally speaking, any serious defect entails the sending of the instrument to the maker, as a special degree of skill is required to execute the necessary repairs. As the indicators are usually not automatic in action, care should be taken in the manipulation not to damage any part, particularly the galvanometer; and it is advisable not to trust the instruments to unskilled observers. The remarks applying to recorders and protecting sheaths in relation to thermo-electric pyrometers (page 92) apply equally in this case.