In Paul’s method of compensation the thermocouple and indicator are placed across a Wheatstone bridge, two arms of which contain resistances of copper, whilst the resistances in the other two arms are of manganin. Any change in temperature at the cold junction is shared by these four resistances, and, whilst affecting the resistance of the copper parts, no change is caused in the manganin parts, as this alloy has a negligible temperature coefficient. If, therefore, the bridge were initially balanced at 20° C., and the temperature rose to 30°, the increased resistance of the copper would destroy the balance, and permit of a small current passing through the indicator. A fall to 10°, by diminishing the resistance of the copper, would cause an equal current to pass through the indicator in the opposite direction. The amount of this current is arranged so as to add the rise in temperature of the cold junction to the reading of the indicator in the one case, and to subtract the fall in the other, thus retaining true readings for the cold-junction temperature at which the couple was standardized. =Constant Temperature Cold Junctions.=—If the cold junction can be kept at a steady temperature, compensators are unnecessary, but no good practical means of achieving this end has yet been devised. Water-cooled heads have already been referred to; but in many situations the connecting-pipes entailed would be objectionable, and hence this arrangement is not greatly used. An alternative method, suggested by Prof. A. Zeleny, is to bury the cold junction in the ground. Recent experiments, conducted at Cambridge by R. S. Whipple, showed that a junction buried 10 feet deep did not vary in temperature by more than 2° C. over a period of three years. This has led to the adoption of buried junctions in special cases; but it is probable that much greater variations would be experienced in the ground beneath large furnaces, in which case the advantages of this procedure would be lost. A common workshop method is to locate the cold junction in a thermos flask filled with oil, when a temperature constant to 2° C. may be secured, although the changes in the temperature of the surrounding atmosphere may be as great as 150 C. For special work, ice may be used in the thermos flask, thus securing absolute constancy; but this procedure is not feasible in ordinary works practice.