When cheap metals are used for the junction the construction may be considerably modified, and often with advantage. In [fig. 6], for example, which represents a thermocouple made by A. Gallenkamp & Co., the metals used are copper and constantan, and the hot junction, fastened by silver solder, is supplemented by a cold junction of the same metals located in the head. The copper wire from the hot junction passes directly to a copper terminal, from whence a copper wire lead is carried to the galvanometer; and the same procedure is carried out with the copper wire from the cold junction, thus realising the circuit shown in [fig. 2]. The cold junction is kept in oil, the temperature of which is registered by a short thermometer, thus enabling (as will be explained later) the correct temperature of the hot junction to be deduced under any circumstances. In this instrument twin-bore fireclay is used to insulate the wires, and the protecting-tube is of iron—which suffices for the upper limit (800° C.) to which the junction may be used. Iron and constantan could be used in this manner by employing iron leads to the galvanometer.
Another type of instrument, rendered practicable by the use of cheap metals, and which may be termed the “heavy type,” is constructed of thick pieces of the metals welded together instead of wires, thus ensuring greater strength and longer life. Messrs Crompton & Co. were the first to introduce thermocouples of this type, consisting of a heavy steel tube, to one end of which a nickel rod is welded, the other end being free, and the length of the rod suitably insulated from the steel tube; leads for the rod and tube being taken to the galvanometer. [Fig. 7] shows a couple of this kind, made by Paul, consisting of an iron tube down the middle of which a constantan rod is passed, insulated from the tube by magnesia. At the tapered end the two metals are welded together, and at the free end a special cap, fitted over the tube and rod, the contact parts being insulated from one another, serves to enable leads to be taken to the galvanometer. Similar thermocouples are made by the Foster Instrument Company ([fig. 8]), and are simple, cheap, and reliable up to 900° C. with an iron-constantan couple, and to 1100° C. with nichrom couples. When worn out they may be replaced, at a trifling cost, by others made from the same batch of metal.
Fig. 7.—Heavy Type, Cheap-metal Pyrometer.
The drawback to the use of carbon as one of the materials for a junction is the difficulty experienced in securing a good contact with the metal with which it is coupled. In nickel-carbon junctions the contact is sometimes ensured by the aid of a spring, which presses the two substances together. Such an arrangement is evidently not so reliable as one in which the materials are welded, and a defective contact, arising from any cause, would lead to serious error. A preferable plan is to screw both the nickel and carbon rods into a cross-piece of either element.
Fig. 8.—Foster’s Cheap-metal Pyrometer.
When applying a thermal junction to the measurement of surface temperatures, such as steam-pipes or the exterior of furnaces, the wires may be passed through a thin disc of metal, about ¼ in. in diameter, and soldered at the back. Suitable materials are copper and constantan, soldered to a thin copper disc with silver solder, and brought to a cold junction in the head of the instrument as shown in [fig. 6]. The terminal piece of the insulation may be made of hard wood, with the holes countersunk so as to cover the solder and enable the wood to touch the disc, which, when pressed on the hot surface, will then rapidly acquire the temperature. The author has found, by trials under varying circumstances, that this method of measuring surface temperatures gives reliable and concordant results. For very high surface temperatures a platinum disc, with one of the usual platinum metal couples soldered to the disc with pure silver, and a piece of twin-bore fireclay brought to the back of the disc, will be found to suffice for most cases arising in practice. A small blowpipe flame is best for soldering the wires to the disc, borax being used as flux in the first case; but no flux is necessary in soldering the platinum metals with pure silver.