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The moving system is pivotted and carried in jewelled centres. The instruments are fitted with air damping to render them “ Dead Beat,” and a soft iron screen to protect them from errors due to " stray fields ” or possible magnetisation of the iron case.

For alternating currents the movement is constructed of laminated iron. For continuous current work the iron is treated so as to be practically free from " Hysteresis error.”

In ammeters the consumption of power is negligible, and there is no temperature error ; in voltmeters the consumption of power is reduced to 6 watts, and the temperature error is very small.

Moving Coil Instruments.—These instruments are constructed on the d’Arsonval principle.

The magnets are treated so as to ensure permanence and a high figure of merit.

The instruments are dead beat, and free from stray field errors.

In the case of voltmeters the scales may be " set up ” ; thus, for example, an instrument for a circuit of 100 volts would read from 80 to 120 volts; more open scales are not recommended.

As in the " soft-iron ” instruments, the cases are usually of black enamelled iron, with nickel or copper relief, and are made in a very large number of types and sizes.

The round type in 5 in., 6 in., 8 in., and 11 in. dials.

3

Sector type Voltmeter. FIG. 2.

The sector type in 5 in., 7 in., 9 in., and 12 in. scales (Fig. 2), and a special type with scale lengths of 17 ins. and 21 ins. ; these can be had with plain or illuminated dials (the 5 in; size is made in plain dial only).

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