Ques. For what service is the D'Arsonval galvanometer adapted? Ans. It is desirable for general use as it is not much affected by changes in the magnetic field. It may be made with high enough period and sensibility to be satisfactory as a ballistic instrument, but for extreme sensibility an instrument of the astatic type is more generally used. Galvanometer "Constant" or "Figure of Merit."--In order that a galvanometer shall be of value as a measuring instrument, the relation between the current and the deflection produced by it must be known. This may be obtained experimentally by determining the value of the current required to produce one scale division. The galvanometer constant then may be defined as _the resistance through which the galvanometer will give a deflection of one scale division when the current applied is at a pressure of one volt_. Accordingly, the deflection as indicated on the scale must be multiplied by its constant or figure of merit, in order to obtain the correct reading. If the scale readings be not directly proportional to the quantity to be measured, the law of the instrument must also be considered. Thus in a tangent galvanometer as previously explained I = K tan [phi] where I = current, [phi] the deflection or scale reading, and K the galvanometer constant. [Illustration: Fig. 537.--Diagram showing method of connecting galvanometer shunt. By the use of a shunt the range of measurement of a galvanometer can be greatly increased.] [Illustration: Fig. 538.--Diagram of a form of universal shunt box for use with galvanometers of widely different resistances. The galvanometer, as indicated at G, is connected across the ends of a series of resistances AB. The main wires are connected, one to end A of the series and the other to a travelling point whose position is varied by means of plugs or by a dial switch.] Galvanometer Shunts.--The sensitiveness of a galvanometer used for measuring current may be reduced to any desired extent by connecting a resistance of known value in parallel with it. Thus, if it be desired to measure a current greater than can be measured directly by the galvanometer, a part of the current can be sent through the resistance or shunt, and the total value of the current calculated. A galvanometer shunt bears a definite ratio to the resistance of the galvanometer, being usually adjusted so that only .1, .01, or .001 part of the current passes through the galvanometer. The degree in which a shunt increases the range of deflection of a galvanometer is called its "multiplying power." [Illustration: Fig. 539.--Ayrton-Mather universal shunt. This shunt may be used with any galvanometer. The total resistance is 10,000 ohms, with shunt powers of 1, 5, 10, 50, 100, 500, and 1,000. It is also fitted with positions in which the galvanometer is shorted and off. The coils are of constantan wire.]