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. considerably, the change in some cases representing over 100° C. on the indicator. A stable condition, due to the relief of strains or other change, was finally reached, and the conclusion drawn that the materials should be thoroughly annealed before calibration. It is desirable in all cases periodically to test the junctions at some standard temperature, and if any conspicuous error be noted, to replace the old junction by a new one.
In addition to the errors due to slow physical changes, a junction may be altered considerably, if imperfectly protected, owing to the chemical action of furnace gases, or of solids with which the junction may come into contact. The vapours of metals such as lead or antimony are very injurious; and platinum in particular is seriously affected by vapours containing phosphorus, if in a reducing atmosphere. So searching is the corrosive action of furnace gases that adequate protection of the junction is essential if errors and damage are to be avoided. When a wire has once been corroded, a junction made with it will not develop the same E.M.F. as before.
Electromotive Force developed by Typical Junctions.—The following table exhibits the E.M.F. generated by several junctions for a range of 100° C., taken at the middle part of the working range in each case. These figures are subject to considerable variation, according to the origin of the metals.
| Couple. | E.M.F. in millivolts for |
| a rise of 100° at middle | |
| of working range. | |
| Platinum-rhodioplatinum (10 per cent. Rh) | 1·1 |
| Platinum-iridioplatinum (10 per cent. Ir) | 1·2 |
| Nickel-constantan | 2·3 |
| Copper-constantan | 5·8 |
| Nickel-copper | 6·1 |
| Iron-constantan | 6·7 |
| Hoskin’s alloys | 7·4 |
It will be noted that the base-metal junctions give much higher values than the platinum series, and hence can be used with a less sensitive, and therefore cheaper, indicator. Base-metal junctions are also, in consequence of the greater E.M.F. furnished, capable of yielding more sensitive readings over a selected range of temperature.
Fig. 4.—Practical Form of Thermo-electric Pyrometer.