4.11 T HE M OVING C OIL G ALVANOMETER · Part 2

no current is flowing through the galvanometer) is in the middle of the scale and not at the left end as shown in Fig. . . Depending on the direction of the current, the pointer deflection is either to the right or the left. The galvanometer cannot as such be used as an ammeter to measure the value of the current in a given circuit. This is for two reasons: (i) Galvanometer is a very sensitive device, it gives a full-scale deflection for a current of the order of µ A. (ii) For measuring currents, the galvanometer has to be connected in series, and as it has a large resistance, this will change the value of the current in the circuit. To overcome these difficulties, one attaches a small resistance r s , called shunt resistance , in parallel with the galvanometer coil; so that most of the current passes through the shunt. The resistance of this arrangement is, R G r s / (R G + r s )  r s if R G >> r s If r s has small value, in relation to the resistance of the rest of the circuit R c , the effect of introducing the measuring instrument is also small and negligible. This FIGURE . The moving coil galvanometer. Its elements are described in the text. Depending on the requirement, this device can be used as a current detector or for measuring the value of the current (ammeter) or voltage (voltmeter).