=The Harris Indicator.=—In the Siemens and Whipple indicators it is necessary, before a reading can be taken, to adjust a resistance until the galvanometer shows no deflection—an operation which takes up time and requires a fair amount of skill. This is obviated in the Harris indicator, made by R. W. Paul, and shown in fig. 37. This instrument is a special form of ohmmeter, which automatically indicates the resistance of the pyrometer by the movement of the pointer; the scale, however, being divided so as to read corresponding temperatures. In this indicator the scale may be made to notify an excess temperature—say 100°—above a given fixed number, and hence is capable of yielding an exact reading over the working range for which it is used. It may also be connected so that the whole scale represents the complete range—say 0° to 1000° C.—or other specified interval. The advantage possessed by this instrument is that the manipulation is much simpler than in the indicators previously described. =The Leeds-Northrup Indicator.=—In this apparatus the Wheatstone bridge principle is employed, but the galvanometer is provided with a scale divided or temperatures. Coils are provided which correspond to an increase of resistance due to a rise of 100° C. on the part of the pyrometer, and by inserting these coils in the circuit the temperature is obtained to the nearest 100°. If the temperature were exactly at an even hundred—say 700°—the pointer of the galvanometer would be at zero on its scale; but if now the temperature rose, the system would no longer be balanced, and the galvanometer pointer would move over its scale by an amount depending upon the potential difference at its terminals. A very sensitive galvanometer would give a movement to the end of its scale with a slight alteration from the correct balance of the system; but by using a coarser instrument the pointer would remain within bounds; and the greater the increase of resistance, the larger would be the deflection. It is possible, in such a case, to divide the galvanometer scale to read temperatures corresponding to a given increase above that of the coils placed in the circuit. In one form of the Leeds-Northrup indicator, the whole scale is thus divided to read 100°, and the reading is obtained by adding the figure shown on the galvanometer to the hundreds represented by the coils inserted. In another form the galvanometer has a central zero, and its scale is divided both right and left, one side giving the number of degrees above, and the other below, the nearest hundred. The observations are thus much simpler than in the case where adjustment to the condition of no deflection is requisite.