General Principles.—When a pure metal is heated, its resistance to electricity increases progressively with the temperature. Certain alloys, on the other hand, show a practically constant resistance at all temperatures, examples of such alloys being constantan, manganin, and platinoid. All the elementary metals, however, exhibit a tangible rise in resistance when the temperature is augmented; and Sir W. Siemens, in 1871, proposed to apply this principle to the measurement of high temperatures by determining the resistance, and deducing the corresponding temperature from a table prepared under known conditions.
The choice of a metal is in this case more greatly restricted than in the selection of materials for a thermal junction. A certain amount of external corrosion does not alter the E.M.F. of a junction; but an alteration in size produces a marked difference in the resistance of a wire, which varies directly as the length and inversely as the area of cross-section. To the necessity for the absence of any internal physical change affecting the resistance is therefore added the further condition of permanence of external dimensions. For temperatures above a red heat the only feasible metals to use are platinum or the more expensive metals of the platinum series—and hence platinum is universally employed for this purpose. The original Siemens pyrometer consisted of 1 metre of platinum wire, 1 millimetre in diameter, wrapped round a porcelain rod, and protected from furnace gases by an iron sheath An elaborate method of measuring the resistance, involving the electrolysis of acidulated water, was adopted for workshop use, but was too involved to become popular. Later, Siemens employed the differential galvanometer method, and finally the Wheatstone bridge, to measure the resistance. Both methods are still in use in connection with resistance pyrometers, and the principle of each will now be explained.
Measurement of Resistance by the Differential Galvanometer.—A differential galvanometer is one which possesses two windings, arranged so that a current passing through the one tends to turn the pointer in one direction, and through the other to cause a movement in the opposite direction. If the currents in each winding simultaneously be equal, the pointer remains at rest under the action of two equal and opposite forces. The experimental attainment of the condition of rest serves as a means of measuring resistance, the circuit being arranged as in [fig. 30]. Current from a battery B passes through a divided circuit, one branch containing the adjustable resistance R and one coil of the galvanometer G; and the other the unknown resistance P and the opposite coil. The resistance R is adjusted until on tapping the key K no deflection on the galvanometer is noted, when the current in each branch of the circuit will be the same. The resistances of each coil of the galvanometer being equal, it follows from Ohm’s law that P is equal to R when no deflection is obtained.
Fig. 30.—Differential Galvanometer Method of Measuring Resistance.
The accuracy of this method depends upon the sensitiveness of the galvanometer, and also upon the extent to which the two coils may be regarded as truly differential, as the measurement evidently assumes complete equality in resistance and effect on the moving part. With modern galvanometers of this pattern, it is possible to secure readings of sufficient accuracy for the purposes of pyrometry. The method, however, is less sensitive than the Wheatstone bridge, now to be described.
Fig. 31.—Principle of Wheatstone Bridge.