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these instrument are widely used in laboratories and switch boards at commercial frequencies because these are cheaper in cost , robust in construction and can be manufactured with required accuracy.

there are two general types of such instrument i.e (i) the attraction type (ii) the repulsion type . the attraction type instrument operate on the principle of repulsion of two adjacent iron piece magnetised by the same magnetic field. repulsion type instrument are are more sensitive as in these instrument large operating torque is developed by having two iron element positioned close together inside the field coil where the magnetizing effect is maximum. in both type of these instrument the current under measurement is passed through a coil of wire. this current carrying coil sets up the necessary field. depending on the magnitude of the current to be measured the coil may be of a few turns of very heavy conductor or of many turns of fine wire. the instrument to be used as an ammeter is provided with a coil of thick wire in order to have low resistance and carry large current and to be used as a voltmeter is provided with a coil of large number of turns of fine wire in order to have high resistance and draws as small current as possible.

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