THERMAL JUNCTIONS USED IN PYROMETERS. ─────────────────────────────────────────────────┬──────────────── │ Upper limit to │ which Junction │ may be used. Couple. ├────────┬─────── │ Deg. │ Deg. │ Cent. │ Fahr. ─────────────────────────────────────────────────┼────────┼─────── Platinum and rhodioplatinum (10 per cent. Rh) │ 1400 │ 2550 2 Rhodioplatinum alloys of different composition │ 1600 │ 2900 Platinum and iridioplatinum (10 per cent. Ir) │ 1100 │ 2000 Nickel and constantan │ 900 │ 1650 Nickel and copper │ 800 │ 1475 Nickel and carbon │ 1000 │ 1850 Nickel and iron │ 1000 │ 1850 Iron and constantan │ 900 │ 1650 Copper and constantan │ 800 │ 1475 Silver and constantan │ 800 │ 1475 2 Nickel chrome alloys of different composition │ │ (Hoskin’s alloys) │ 1100 │ 2010 Nickel-chrome alloy and nickel-aluminium alloy │ 1100 │ 2010 2 Iron-nickel alloys of different composition │ 1000 │ 1850 ─────────────────────────────────────────────────┴────────┴─────── The electromotive force developed by a junction of any given pair of metals when heated to a given temperature varies according to the origin of the metals. It is not unusual, for example, for two samples of 10 per cent. rhodioplatinum, obtained from different sources, to show a difference in this respect of 40 per cent. when coupled with the same piece of platinum. Equal or greater divergences may be noted with other metals; and hence the replacement of a junction can only be effected, with accuracy, by wires from the same lengths of which the junction formed a part. As showing how platinum itself is not uniform, it may be mentioned that almost any two pieces of platinum wire, if not from the same length, will cause a deflection on a sensitive galvanometer when made into a junction and heated. It is therefore customary for makers to obtain considerable quantities of wire of a given kind, homogeneous as far as possible, in order that a number of identical instruments may be made, and the junctions replaced, when necessary, without alteration of the scale of the indicator.