=Siemens’ Differential Indicator.=—This form of indicator is still in use, and consists of a differential galvanometer and box of resistance coils, connected as shown in fig. 30. By adjusting the coils until no deflection is produced, the resistance of the pyrometer is obtained, and the corresponding temperature read off from tables provided. This form of indicator is preferred by some users, but it is less sensitive than the more recent Wheatstone bridge indicator made by this firm (fig. 35), and equally difficult to manipulate. =Recorders for Resistance Pyrometers.=—The value of records in high-temperature work has led to the invention of recording mechanisms for use with resistance pyrometers. The form in common use in Britain is that devised by Callendar, shown in fig. 38, and consists of a mechanism for restoring automatically the balance of the resistances in a Wheatstone bridge circuit, in such a manner as to indicate the existing resistance on a chart. To this end the moving coil of the galvanometer carries a boom, or contact-arm, which, on swinging to the right or left, completes one of two electric circuits. The closing of either circuit brings into action a clockwork mechanism, which causes a slider carrying a pen to move over the bridge wire until the balance is restored, and incidentally to produce a mark in ink on a paper wound on a drum, which rotates at a known speed. When the resistance of the pyrometer is balanced, the galvanometer boom will be in a central position, and the slider at rest; whereas a rise in temperature causing an increase in the resistance of the pyrometer, will result in the boom swinging over and completing the circuit, which introduces more resistance in opposition to the pyrometer. A fall in temperature will similarly result in the liberation of the second mechanism, owing to the boom swinging in the opposite direction, with the result that the slider moves so as to oppose a less resistance to the pyrometer. If the chart be divided horizontally into equal spaces, representing equal increments or decrements of resistance, they may be marked to represent degrees on the platinum scale, which may be translated into ordinary degrees by reference to a conversion table. In careful and skilled hands this recorder gives excellent results, and the value of the records obtained is clearly shown by an inspection of the example shown in fig. 39, which represents the fluctuations of an annealing furnace during a period of nine hours. It will be noted that during the period covered by workman A the furnace has received constant and careful attention; but workman B has evidently neglected his duty conspicuously at two separate times. [Illustration: FIG. 38.—CALLENDAR’S RECORDER.] [Illustration: FIG. 39.—RECORD OBTAINED WITH CALLENDAR’S RECORDER.]