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The Midget pattern (Figs. 15 and 16), a smaller switch, suitable for voltages up to 80 volts, and currents up to 50 amperes makes it suitable for low voltage installations for metal filament lamps. It differs from the Crawley type in that it possesses a permanent magnet to induce the magnetism in the soft iron bar attached to the dipper bar instead of a coil connected across the cells ; in other respects the principle is the same. It is of thp enclosed type, the mercury cups and terminals being fitted to the front of the case.

MAIN SWITCH

Fig.16.

DYNAMO

Fig. 15.

Insulation Testing-Set 11 the Ohmer ” (Cox’s Patent.)—Consists of an ohmmeter and generator mounted in one case. The ohmmeter is of the electrostatic type, and is thus independent of external fields, the moving system consists of a number of parallel vanes of mica coated with aluminium, giving great rigidity and lightness. These move into a series of fixed vanes or “inductors,” on the principle of an electrostatic voltmeter. Electro-magnetic damping is provided, rendering the instrument dead-beat.

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