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. ──────────────────────────────────────────┬───────────────────────── │ E.M.F. in millivolts for Couple. │ 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. [Illustration: FIG. 4.—PRACTICAL FORM OF THERMO-ELECTRIC PYROMETER.]