=Siemens’ Indicator.=—This instrument is based upon the Wheatstone bridge principle, and is shown in fig. 35. The galvanometer is mounted in the centre of the dial, round the edge of which is fixed a ring on which the adjustable resistance is wound in spiral form. Suitable terminals are provided, duly labelled, to which the battery, pyrometer leads, and compensator are attached. A brass arm, movable about the centre of the dial, terminates in a tapping-key which moves over the adjustable resistance; the key being placed in the battery circuit. The fixed known resistances are located in the interior of the indicator. The adjustment consists in moving the key round the circumference until, on tapping, no deflection is obtained on the galvanometer. The pointed end of the movable arm then indicates the temperature of the pyrometer on the dial, which is marked in temperatures corresponding to the resistance opposed to the pyrometer for different positions of the key. In taking a reading, the operator is guided by the fact that when the temperature indicated is too high, the movement of the galvanometer needle will be in one direction; whereas if too low an opposite deflection will be given. The intermediate position of no deflection must then be found by trial; and the procedure should not occupy more than two minutes if the observer possess an approximate notion of the temperature to be measured. [Illustration: FIG. 36—WHIPPLE’S INDICATOR.] =Whipple’s Indicator.=—This instrument (fig. 36) is employed by the Cambridge and Paul Instrument Company, and is also a form of Wheatstone bridge. The pyrometer leads and compensator are connected to properly labelled terminals T, and the battery to other terminals at the opposite side of the box. The pointer of the galvanometer is visible through the small window B, and a battery of two dry cells is placed at the side of the box. The fixed resistances are contained in the interior, and the adjustable resistance consists of a continuous wire wound on a drum, which may be rotated by the handle H. The shaft connecting H with the drum is screwed, and works in a nut, so that the turning of H produces a spiral movement of the drum. The adjustment consists in rotating H until, on tapping the key F, no deflection of the galvanometer pointer is observed. The temperature of the pyrometer is then read off directly from a paper scale wound round the drum and rotating with it, visible through the window A, the reading being indicated by a fixed pointer. This arrangement forms a compact and convenient indicator. [Illustration: FIG. 37.—THE HARRIS INDICATOR.]