Electro-magnets, Interlocking.
Electro-magnets so arranged that their armatures interlock. Thus two magnets, A A and B B, may be placed with their armatures, M and N, at right angles and both normally pulled away from the poles. When the armature M is attracted a catch on its end is retained by a hole in the end of the other armature N, and when the latter armature N is attracted by its magnet the armature M is released. In the mechanism shown in the cut the movements of the wheel R are controlled. Normally it is held motionless by the catch upon the bottom of the armature M, coming against the tooth projecting from its periphery. A momentary current through the coils of the magnet A A releases it, by attracting M, which is caught and retained by N, and leaves it free to rotate. A momentary current through the coils of the magnet B B again releases M, which drops down and engages the tooth upon R and arrests its motion.
Fig. 151. INTERLOCKING ELECTRO-MAGNETS.
222 STANDARD ELECTRICAL DICTIONARY.
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 pa.s.s 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:
F = ( ( V - V1 )^2 * S ) / ( 8 * PI * d^2 )
223 STANDARD ELECTRICAL DICTIONARY.
If V = 0 this reduces to
F = ( V^2 * S ) / ( 8 * PI * d^2 ) (2) or V = d * SquareRoot( (8 * PI * F ) / S ) (3)
As F is expressed as a weight, and S and a as measures of area and length, this gives a means of directly obtaining potential values in absolute measure. (See Idiostatic Method--Heterostatic Method.)
Synonyms--Attracted Disc Electrometer--Weight Electrometer.
Fig. 152. SECTION OF BASE OF PORTABLE ELECTROMETER.
In some forms the movable disc is above the other, and supported at the end of a balance beam. In others a spring support, arranged so as to enable the attraction to be determined in weight units, is adopted. The cuts, Figs. 152 and 154, show one of the latter type, the portable electrometer. The disc portion is contained within a cylindrical vessel.
Fig. 153. DIAGRAM ILl.u.s.tRATING THEORY OF ABSOLUTE ELECTROMETER.
Referring to Fig. 152 g is the stationary disc, charged through the wire connection r; f is the movable disc, carried by a balance beam poised at i on a horizontal and transverse stretched platinum wire, acting as a torsional spring. The position of the end k of the balance beam shows when the disc f is in the plane of the guard ring h h. The end k is forked horizontally and a horizontal sighting wire or hair is fastened across the opening of the fork. When the hair is midway between two dots on a vertical scale the lever is in the sighted position, as it is called, and the disc is in the plane of the guard ring.
224 STANDARD ELECTRICAL DICTIONARY.
Fig. 154. PORTABLE ELECTROMETER.
The general construction is seen in Fig. 154. There the fixed disc D is carried by insulating stem g1. The charging electrode is supported by an insulating stem g2, and without contact with the box pa.s.ses out of its cover through a guard tube E, with cover, sometimes called umbrella, V.
The umbrella is to protect the apparatus from air currents. At m is the sighting lens. H is a lead box packed with pumice stone, moistened with oil of vitriol or concentrated sulphuric acid, to preserve the atmosphere dry. Before use the acid is boiled with some ammonium sulphate to expel any corrosive nitrogen oxides, which might corrode the bra.s.s.
In use the upper disc is charged by its insulated electrode within the tube E; the movable disc is charged if desired directly through the case of the instrument. The upper disc is screwed up or down by the micrometer head M, until the sighted position is reached. The readings of the micrometer on the top of the case give the data for calculation.
225 STANDARD ELECTRICAL DICTIONARY.
Fig. 155. LIPPMAN"S CAPILLARY ELECTROMETER.
Electrometer, Capillary.
An electrometer for measuring potential difference by capillary action, which latter is affected by electrostatic excitement. A tube A contains mercury; its end drawn out to a fine aperture dips into a vessel B which contains dilute sulphuric acid with mercury under it, as shown. Wires running from the binding-posts a and b connect one with the mercury in A, the other with that in B. The upper end of the tube A connects with a thick rubber mercury reservoir T, and manometer H. The surface tension of the mercury-acid film at the lower end of the tube A keeps all in equilibrium. If now a potential difference is established between a and b, as by connecting a battery thereto, the surface tension is increased and the mercury rises in the tube B. By s.c.r.e.w.i.n.g down the compressing clamp E, the mercury is brought back to its original position. The microscope M is used to determine this position with accuracy. The change in reading of the manometer gives the relation of change of surface tension and therefore of potential. Each electrometer needs special graduation or calibration, but is exceedingly sensitive and accurate. It cannot be used for greater potential differences than .6 volt, but can measure .0006 volt. Its electrostatic capacity is so small that it can indicate rapid changes. Another form indicates potential difference by the movement of a drop of sulphuric acid in a horizontal gla.s.s tube, otherwise filled with mercury, and whose ends lead into two mercury cups or reservoirs. The pair of electrodes to be tested are connected to the mercury vessels. The drop moves towards the negative pole, and its movement for small potential differences (less than one volt) is proportional to the electro-motive force or potential difference.
226 STANDARD ELECTRICAL DICTIONARY.
Electrometer Gauge.
An absolute electrometer (see Electrometer, Absolute) forming an attachment to a Thomson quadrant electrometer. It is used to test the potential of the flat needle connected with the inner surface of the Leyden jar condenser of the apparatus. This it does by measuring the attraction between itself and an attracting disc, the latter connected by a conductor with the interior of the jar.
Electrometer, Lane"s.
A Leyden jar with mounted discharger, so that when charged to a certain point it discharges itself. It is connected with one coating of any jar whose charge is to be measured, which jar is then charged by the other coating. As the jar under trial becomes charged to a certain point the electrometer jar discharges itself, and the number of discharges is the measure of the charge of the other jar. It is really a unit jar, q. v.
Fig. 156. THOMSON"S QUADRANT ELECTROMETER.