The alloy known as “constantan,” which figures largely in the foregoing table, is composed of nickel and copper, and is practically identical with the alloy sold as “Eureka” or “Advance.” It has a high specific resistance, and a very small temperature coefficient, and is much used for winding resistances. Couples formed of constantan and other metals furnish on heating an E.M.F. several times greater than that yielded by couples of the platinum series, and show an equally steady rise of E.M.F. with temperature. This alloy has proved of great service in connection with the thermo-electric method of measuring temperatures. Couples formed of nickel-chrome alloys, known as “Hoskin’s alloys,” have been introduced into Britain by the Foster Instrument Company, which may be used continuously to 1100° C., and for occasional readings up to 1300° C. Another couple, much used in America, consists of an alloy of 90 per cent. nickel and 10 per cent. chromium, and an alloy of 98 per cent. nickel and 2 per cent. aluminium, which may be used up to 1100° C. Other couples, formed of alloys of nickel, chromium, iron, aluminium, etc., have been introduced by different makers, but have not proved so satisfactory as those mentioned above. =Changes in Thermal Junctions when constantly used.=—No metal appears to be able to withstand a high temperature continuously without undergoing some physical alteration; and for this reason the E.M.F. developed by a given junction is liable to change after a period of constant use. At temperatures above 1100° C., platinum, for example, undergoes a notable change in a comparatively short period, but below 1000° C., the change is very slight, and if this range be not exceeded, a platinum-rhodioplatinum or iridioplatinum junction may be used for years without serious error arising from this cause. This liability to change is one of the factors which restricts the range of thermal junctions, which should never be used continuously beyond the temperature at which the alteration commences to become large. A second cause of discrepancy is the possible alteration in the composition of an alloy, due to one of the constituents leaving in the form of vapour, as is noted with iridioplatinum alloys, from which the iridium volatilises in tangible quantities above 1100° C., causing a fall of 10 per cent. or more in the thermo-electric value of the junction of these alloys with platinum. Constantan appears to be very stable in its thermo-electric properties, and the various junctions in which it plays a part show a high degree of stability if not overheated. Rhodioplatinum alloys are very stable, and for temperatures exceeding 1100° C. a junction of two of these alloys, of different composition, is more durable than one in which pure platinum is used. An extended series of tests on base-metal junctions made in America by Kowalke showed that continuous heating of couples as received from the makers altered the E.M.F.