=Cold Junction Compensators.=—The necessity for paying attention to the cold junction has led to various attempts to compensate automatically for changes of temperature at this part of the pyrometer. A thermometer located near the cold junction, as in fig. 6, is all that is needed to correct a two-junction circuit; but when a three-junction circuit is used a correct reading is not secured by adding the excess temperature of the thermometer over the calibration temperature to the reading on the indicator. In Bristol’s arrangement a mercury thermometer, with a large bulb and wide stem, is stationed at the cold junction, and participates in any temperature change. In the stem is placed a loop of thin platinum wire, which forms part of the pyrometer circuit. When the mercury is heated it expands up the stem and short-circuits a portion of the loop, thereby diminishing the resistance of the pyrometer circuit, and tending to increase the deflection on the indicator. Simultaneously the cold junction will be heated, tending to diminish the current, and so to cause a less deflection. By adjustment these two tendencies may be counterbalanced, so that the reading is unaffected, but such adjustment will only apply to a given E.M.F., and therefore to one temperature of the hot junction. Hence this method fails in general application. Peake’s compensated leads are intended to remedy cold-junction errors by transferring this junction, in effect, to the galvanometer. They are used for pyrometers in which the platinum metals are employed, and consist of wires of two different alloys of copper and nickel, which connect the cold end to the indicator. These alloys are such that the electromotive forces set up at the junctions in the head—Pt and Cu-Ni 1, and Pt-Ir with Cu-Ni 2—are equal and opposite at all working temperatures, and hence changes at the cold junctions do not affect the reading. At the indicator, however, temperature changes would cause an alteration in deflection; but as the indicator is generally placed well away from the furnace, and is not liable to notable heating or cooling, the possible errors are greatly reduced by the use of these leads. They are obviously of no value for use with base-metal pyrometers, as the wires used in such may be prolonged to the indicator, with an identical result. [Illustration: FIG. 17.—DARLING’S COMPENSATOR, FITTED TO GALVANOMETER.]