Fig. 3.—Three-junction Thermo-electric Circuit.
A point of practical utility in thermo-electric work is the fact that if a wire be interrupted by a length of other metal, as indicated at C in [fig. 3], no current will be set up in a circuit if both joints are equally heated, as the electromotive forces generated at each junction are in opposition. It is thus possible to interrupt a circuit by a plug-key or switch, without introducing an error; always provided that an even temperature prevails over the region containing the joints.
Another useful fact is that if two wires be brought into contact, they may be fastened over the joint by soldering or using a third metal, without alteration of thermo-electric value, except in rare cases. Thus a copper-constantan or iron-constantan junction may suitably be united by silver solder, using borax as a flux, thus avoiding the uncertainty of contact which must always occur when the wires are merely twisted together. Welding, however, is preferable to soldering.
Metals used for Thermal Junctions.—Until recent years it was customary to employ a platinum-rhodioplatinum or platinum-iridioplatinum junction for all temperatures beyond the scope of the mercury thermometer. The almost prohibitive price of these metals has caused investigations to be made with a view to discovering cheaper substitutes, with successful results up to 1000° C. or 1800° F., thus comprehending the range of temperatures employed in many industrial processes. Above this temperature the platinum series of metals are still used for accurate working, but it will be of great advantage if the range measurable by cheap or “base” metals can be further extended. Promise in this direction is afforded by the properties of fused metals when used in thermal junctions. An investigation by the author has shown that in general the E.M.F. developed by a junction does not undergo any sudden change when one or both metals melt, but continues as if fusion had not occurred. By making arrangements to maintain the continuity of the circuit after fusion, it may be possible to read temperatures approximating to the boiling points of metals such as copper and tin, both of which are over 2000° C. The base metals are not so durable as platinum and kindred metals, but as the cost of replacement is negligible, this drawback is of little importance. Moreover, base-metal junctions usually develop a much higher E.M.F. than the platinum metals, which enables stronger and cheaper galvanometers to be used as indicators.
Thermal Junctions used in Pyrometers.
| Couple. | Upper limit to which | |
| Junction may be used. | ||
| 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.