An automatic compensator for use with base-metal pyrometers has been devised by the author, and is illustrated in figs. 17 and 18. A spiral made of a compound strip of two metals is attached to the needle of the indicator, and coils or uncoils when cooled or heated, thereby moving the pointer over the scale. The length of the spiral is such that an alteration of a given number of degrees in its temperature moves the pointer by the same number of degrees on the scale—or, in other words, the temperature scale of the pyrometer is identical with that of the spiral. The metals forming the junction are continued, in the form of wires, to the interior of the galvanometer, where a cold junction is formed, which will always possess the same temperature as the spiral. The scale is constructed to represent differences of temperature between the hot and cold junctions, and before coupling up the pyrometer the pointer indicates the temperature of the spiral; that is, of the cold junction. On connecting the thermocouple the pointer is moved by the coil of the indicator through an amount represented by the difference in temperature between the two junctions, and therefore finally indicates the temperature of the hot junction. [Illustration: FIG. 18.—INDICATOR FITTED WITH DARLING’S COMPENSATOR.] _Example._—If the cold junction were at 20°, the pointer, before connecting the couple, would indicate 20° on the scale. If the hot junction were 580° hotter than the cold, then on completing the circuit the pointer would move 580 additional degrees along the scale, so that the figure indicated would be (20 + 580) = 600°, the temperature of the hot junction. If now the indicator were heated by 10°, the spiral would tend to augment the deflection by 10°, but simultaneously the deflection due to the junctions would fall off by 10°, and the reading would still be 600°. This method of compensation renders the readings independent of the cold junction, and, in addition to its use for high temperatures, enables ordinary and low temperatures to be read simply and correctly, as will be shown later. The spiral is located in the tower rising from the top of the indicator in fig. 18.