In certain manufacturing processes it is necessary to determine the temperature of furnaces. Hence the need of some instrument that is simple, accurate, and capable of being handled by a workman without special mechanical or electrical knowledge. Such an instrument is found in the electric pyrometer (Fig. 81) which consists of a thermo-ele-ment, or insertion tube, for exposure to the heat, and a sensitive galvanometer to indicate the temperature at a convenient distance from the source of heat.
The principle underlying this pyrometer is that when any metal is heated an electric current is set up. The intensity of the current depends upon the temperature to which the metal is heated. Thus, measuring the current measures also the temperature at the extremities of the metal.
Fig. 81. - Pyrometer.
The lower portion of the thermo-element, which is inserted into the metal, is protected by crucible material (a clay substance that will resist great heat) or by a tube of pure graphite with an insertion of quartz glass. In the latter case, the graphite protection can only be 8 in. long, whereas in the former case (for temperatures up to 2370° F.), the protection tube for the thermo-element can be any desired length. The latter is particularly valuable in cases where the increase of temperature has to be watched while the crucible is in the oven, so that it can be lifted out at the correct moment.
The thermo-element consists essentially of two wires or rods of different materials, which are joined or fused together at their extreme ends and exposed to the heat. These ends are called the hot junction. The other extremes of the rods are called the cold junction. The cold junction projects into the open air and is connected to the leading wires of the galvanometer by means of screws.
The two rods of the thermo-element are of different electrical conductivity. If, therefore, the ends of the rods at the hot junction are heated, a difference of potential is produced, causing an electric current to flow, varying in strength with the degree of the thermal difference between the cold and the hot junctions, or with the intensity of the heat to which the thermo-element is exposed. The relation of this current to the temperature has been determined accurately by experiment, and the scale of the galvanometer can there-fore be divided to read directly in Fahrenheit or Centigrade degrees. Thus, as soon as the thermo-element is exposed to heat or cold, the electric pressure or current produced in the two rods actuates the mechanism of the galvanometer, and the needle of the latter indicates directly the exact temperature of the hot junction at the place where the thermo-element is inserted.
Inasmuch as the electric current produced in the thermo-element through the heating of the hot junction depends on the difference between the temperature at the two extremes of the rods, it is, of course, essential that the outer ends of the rods or the cold junction be kept cool.
The insertion tubes are made in various lengths and fitted with protection tubes and flanges (screwed couplings) to adapt them exactly to the different processes or apparatus for which they are required. The constituents of the thermoelement vary according to the intensity of the heat for which they are intended. For temperatures up to 1100° F. or 600° C, the element consists of nickel and a special metal alloy; for temperatures up to 2300° F. or 1250° C, nickel and a special carbon are used; while for temperatures up to 2900° F. or 1600° C, platinum and platinum rhodium give the best results.