A glass plate fixed to a wooden or metal shaft is rotated by a winch.
The whole organ can be rotated by special muscles.
Cayley's screws were peculiar, inasmuch as they were superimposed and rotated in opposite directions.
In front of them a disk of ebonite or glass, having carriers of metal fixed to its edge, was rotated by a winch.
In most of the systems that have been proposed this result is attained by electrical regulation; in one, however, a mechanical method is adopted, the dynamo being so' hung that it allows the driving belt to slip when the speed of the axle exceeds a certain limit, the armature thus being rotated at an approximately constant speed.