Returning to the junction, it is advisable always to weld the wires, and not to rely upon the contact resulting from twisting them together. Platinum and the platinum alloys may be welded readily by placing the junction in a coal-gas blowpipe fed with oxygen instead of air. For work at lower temperatures the platinum metals may be soldered by means of a small quantity of gold, in the flame of a Bunsen burner. 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. [Illustration: FIG. 7.—HEAVY TYPE, CHEAP-METAL PYROMETER.]