Electro-magnet, Stopped Coil.
An electro-magnet consisting of a tubular coil, in which a short fixed
core is contained, stopping up the aperture to a certain distance, while
the armature is a plunger entering the aperture. This gives a longer
range of action than usual.
Electro-magnet, Surgical.
An electro-magnet, generally of straight or bar form, fitted with
different shaped pole pieces, used for the extraction of fragments of
iron or steel from the eyes. Some very curious cases of successful
operations on the eyes of workmen, into whose eyes fragments of steel or
iron had penetrated, are on record.
Electro-medical Baths.
A bath for the person provided with connections and electrodes for
causing a current of electricity of any desired type to pass through the
body of the bather. Like all electro-therapeutical treatment, it should
be administered under the direction of a physician only.
Electro-metallurgy.
(a) In the reduction of ores the electric current has been proposed but
never extensively used, except in the reduction of aluminum and its
alloys. (See Reduction of Ores, Electric.)
(b) Electro-plating and deposition of metal from solutions is another
branch. (See Electroplating and Electrotyping.)
(c) The concentration of iron ores by magnetic attraction may come under
this head. (See Magnetic Concentration of Ores.)
Electrometer.
An instrument for use in the measurement of potential difference, by the
attraction or repulsion of statically charged bodies. They are
distinguished from galvanometers as the latter are really current
measurers, even if wound for use as voltmeters, depending for their
action upon the action of the current circulating in their coils.
Electrometer, Absolute.
An electrometer designed to give directly the value of a charge in
absolute units. In one form a plate, a b, of conducting surface is
supported or poised horizontally below a second larger plate C, also of
conducting surface. The poised plate is surrounded by a detached guard
ring--an annular or perforated plate, r g r' g'--exactly level and even
with it as regards the upper surface. The inner plate is carried by a
delicate balance. In use it is connected to one of the conductors and
the lower plate to earth or to the other. The attraction between them is
determined by weighing. By calculation the results can be made absolute,
as they depend on actual size of the plates and their distance, outside
of the potential difference of which of course nothing can be said. If S
is the area of the disc, d the distance of the plates, V-V1 the
difference of their potential, which is to be measured, and F the force
required to balance their attraction, we have: