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. [Illustration: 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. In deciding upon the length of a thermocouple it must be remembered that the temperature recorded is that prevailing in the region of the hot junction. When the temperature of a furnace is uniform it is sufficient to allow the end of the thermocouple to protrude about 12 inches into the interior, but when following the change of temperature undergone by objects in a furnace the end must be located near the objects. If the distance from the exterior of the furnace to the objects exceed 2 feet, the thermocouple should be inserted through the roof so as to hang vertically, as if placed through the side it would droop by its own weight at high temperatures. The distance between the exterior of the furnace and the cold junctions should be at least 15 inches in all cases in which the heating of the cold junction is not automatically compensated. After inserting the couple the opening through the furnace wall should be closed by means of suitable luting-clay.