=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. [Illustration: 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. [Illustration: FIG. 31.—PRINCIPLE OF WHEATSTONE BRIDGE.] =Measurement of Resistance by the Wheatstone Bridge.=—The principle of this method is shown in fig. 31, where _a_ and _b_ are two fixed resistances of known value; _d_ is an adjustable resistance; _x_ the resistance to be measured; B a battery; and G a sensitive galvanometer. If, in this circuit, _d_ be adjusted until no deflection is shown on the galvanometer, then _a_/_b_ = _x_/_d_; or _x_ = (_a_ × _d_)/_b_. Hence, if _a_ = _b_, then _x_ will be equal to _d_. It is not difficult to construct a portable apparatus, suitable for workshop use, by means of which the value of _x_ may be determined to 0·01 ohm; and in the laboratory, with a very delicate galvanometer, 0·001 ohm may readily be detected. The Wheatstone bridge method is the best for the accurate measurement of resistance; but in resistance pyrometers it is sometimes advisable to sacrifice extreme accuracy in order to gain advantages in other directions, as will be shown subsequently.