Fig. 300. WIRE GAUZE ELECTRIC SCREEN.
Screen, Electric.
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.
Secondary Actions.
In electrolysis the direct products of the electrical decomposition are
not always obtained at the electrodes, but products due to their
reaction on the water and other chemicals may appear. These constitute
secondary actions. Thus if a solution of copper sulphate is electrolyzed
with platinum electrodes, metallic copper appears at one pole and
sulphuric acid and oxygen gas at the other. But the products of
electrolysis by the current are copper (Cu) and sulphion (SO4). The
latter reacting on water sets free oxygen gas and forms sulphuric acid.
The latter is a secondary action.
Secondary Generator.
(a) An alternating current converter generating a so-called secondary
current.
(b) A secondary battery, q. v., may be thus termed.
Secondary, Movable.
The term movable secondaries has been applied to rings, spheres and
discs of conducting material, such as copper, whose behavior when near
the pole of an electro-magnet traversed by an alternating current, have
been studied by Elihu Thomson. Such masses are subjected to very
peculiar movements and mutual reactions. As the phenomena are due to
induced currents the above term has been applied to the masses in which
the currents are induced.