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Electric Screen

A large plate or a hollow case or cage of conducting material connected with the earth, and used to protect any body placed within it from electrostatic influences.

If within a hollow conducting sphere an electrified body is placed, the inner surface of the sphere will be charged with electricity of opposite kind to that of the sphere, and the outer surface with the same kind as that of the sphere. Thus the sum of the electricities called into action by induction is zero. The two inner charges are bound to each other. The induced charge on the outer surface of the sphere is all that has any effect on objects in the outer air.

If the outer surface is connected to the earth it becomes discharged, and however highly electrified the body introduced into the sphere and the inner surface of such sphere may be, they produce no external effects, as they are bound one to the other.

If the sphere is connected to the earth and an unelectrified object is placed within it, such object will be perfectly shielded from the effects of an outer electrostatic field. Perforated tinfoil or wire gauze has just as good a result. A large plate of metal connected to the earth has the same effect. The screen whether plane or hollow simply retains a bound charge due to the field of force, thereby neutralizing it, and the electricity of the opposite sign escapes to the earth. Thus a true shielding or screening effect is produced.

In the cut an experiment is shown in which an electric screen is carried by a Leyden jar. Pith balls are suspended outside and inside of it. By the approach of an electrified body the outer pith balls will diverge, while no effect is produced upon the inner ones.