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cove i descends, thus shifting the brushes the opposite way andincreasing the current to the normal strength. The dash-potconnected to the iron core 1 may be of ordinary construction ;but it is better, especially in machines for arc lights, to providethe piston of the dash-pot with a valve, as indicated in the dia-grams. This valve permits a comparatively easy downward move-ment of the iron core, but checks its movement when it is drawnup by the solenoid. Such an arrangement has the advantagethat a great number of lights may be put on without diminishingthe light-power of the lamps in the circuit, as the brushes assumeat once the proper position. When lights are cut, out, the dash-pot acts to retard the movement; but if the current is considerablyincreased the solenoid gets abnormally strong and the brushesare shifted instantly. The regulator being properly adjusted,lights or other devices may be put on or out with scarcely anyperceptible difference. It is obvious that instead of the dash-potany other retarding device may be used.