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156 through the coil and observing the resulting deflection, the instrument can be calibrated.

For the measurement of heavy currents such as are used in lighting or power work the dynamic force between a current and a magnetic field may also be utilised, but in a somewhat different way. Fig. 8 is a diagrammatic representation of the principles on which such instruments are constructed. Within the polar cavity of a permanent steel magnet N S is placed an armature A, and into the ring-shaped space between the two is inserted a coil C formed of fine wire and delicately pivoted in the centre of the armature. To the coil is attached a pointer suitably counter-weighted. The coil is under the control of a spiral spring S which keeps it normally in the position shown in the sketch, the pointer standing at zero. The current to be measured I is flowing along the wire c d, and into this is inserted a resistance R having a definite and known ratio to the resistance of the coil C. According to this ratio more or less of the current I will be deflected and carried through the coil of the instrument by means of the attachment of flexible wires to the terminals a b of the coil, and it is obvious that by changing the resistance R one and the same