of 50 to 200 volts by means of a dry pile or voltaic battery, or from a lighting circuit. To facilitate the communication of the charge to the needle, the quartz fibre and its attachments are rendered conductive by a thin film of solution of hygroscopic salt such as calcium chloride. The lightness of the needle enables the instrument to be moved without fear of damaging the suspension. The upper end of the quartz fibre is rotated by a torsion head, and a metal cover serves to screen the instrument from stray electrostatic fields. With a quartz fibre 0.009 mm. thick and 60 mm. long, the needle being charged to 110 volts, the period and swing of the needle was 18 seconds. With the scale at a distance of two metres, a deflection of 130 mm. was produced by an electromotive force of 0.1 volt. By using a quartz fibre of about half the above diameter the sensitiveness was much increased. An instrument of this form is valuable in measuring small alternating currents by the fall of potential produced down a known resistance. In the same way it may be employed to measure high potentials by measuring the fall of potential down a fraction of a known non-inductive resistance. In this last case, however, the capacity of the electrometer used must be small, otherwise an error is introduced.[1]
See, in addition to references already given, A. Gray, Absolute Measurements in Electricity and Magnetism (London, 1888), vol. i. p. 254; A. Winkelmann, Handbuch der Physik (Breslau, 1905), pp. 58-70, which contains a large number of references to original papers on electrometers. (J. A. F.)
ELECTRON, the name suggested by Dr G. Johnstone Stoney
in 1891 for the natural unit of electricity to which he had drawn
attention in 1874, and subsequently applied to the ultra-atomic
particles carrying negative charges of electricity, of which
Professor Sir J. J. Thomson proved in 1897 that the cathode
rays consisted. The electrons, which Thomson at first called
corpuscles, are point charges of negative electricity, their inertia
showing them to have a mass equal to about 12000 that of
the hydrogen atom. They are apparently derivable from all
kinds of matter, and are believed to be components at any rate
of the chemical atom. The electronic theory of the chemical
atom supposes, in fact, that atoms are congeries of electrons
in rapid orbital motion. The size of the electron is to that of an
atom roughly in the ratio of a pin’s head to the dome of St
Paul’s cathedral. The electron is always associated with the unit
charge of negative electricity, and it has been suggested that
its inertia is wholly electrical. For further details see the
articles on Electricity; Magnetism; Matter; Radioactivity;
Conduction, Electric; The Electron Theory, E.
Fournier d’Albe (London, 1907); and the original papers of
Dr G. Johnstone Stoney, Proc. Brit. Ass. (Belfast, August 1874),
“On the Physical Units of Nature,” and Trans. Royal Dublin
Society (1891), 4, p. 583.
ELECTROPHORUS, an instrument invented by Alessandro
Volta in 1775, by which mechanical work is transformed into
electrostatic charge by the aid of a small initial charge of electricity.
The operation depends on the facts of electrostatic induction
discovered by John Canton in 1753, and, independently,
by J. K. Wilcke in 1762 (see Electricity). Volta, in a letter
to J. Priestley on the 10th of June 1775 (see Collezione dell’ opere,
ed. 1816, vol. i. p. 118), described the invention of a device
he called an elettroforo perpetuo, based on the fact that a conductor
held near an electrified body and touched by the finger
was found, when withdrawn, to possess an electric charge of
opposite sign to that of the electrified body. His electrophorus
in one form consisted of a disk of non-conducting material, such
as pitch or resin, placed between two metal sheets, one being
provided with an insulating handle. For the pitch or resin
may be substituted a sheet of glass, ebonite, india-rubber or
any other good dielectric placed upon a metallic sheet, called
the sole-plate. To use the apparatus the surface of the dielectric
is rubbed with a piece of warm flannel, silk or catskin, so as to
electrify it, and the upper metal plate is then placed upon it.
Owing to the irregularities in the surfaces of the dielectric and
upper plate the two are only in contact at a few points, and owing
to the insulating quality of the dielectric its surface electrical
charge cannot move over it. It therefore acts inductively upon
the upper plate and induces on the adjacent surface an electric
charge of opposite sign. Suppose, for instance, that the dielectric
is a plate of resin rubbed with catskin, it will then be negatively
electrified and will act by induction on the upper plate across
the film of air separating the upper resin surface and lower
surface of the upper metal plate. If the upper plate is touched
with the finger or connected to earth for a moment, a negative
charge will escape from the metal plate to earth at that moment.
The arrangement thus constitutes a condenser; the upper plate
on its under surface carries a charge of positive electricity and
the resin plate a charge of negative electricity on its upper
surface, the air film between them being the dielectric of the
condenser. If, therefore, the upper plate is elevated, mechanical
work has to be done to separate the two electric charges. Accordingly
on raising the upper plate, the charge on it, in old-fashioned
nomenclature, becomes free and can be communicated
to any other insulated conductor at a lower potential, the upper
plate thereby becoming more or less discharged. On placing
the upper plate again on the resin and touching it for a moment,
the process can be repeated, and so at the expense of mechanical
work done in lifting the upper plate against the mutual attraction
of two electric charges of opposite sign, an indefinitely large
electric charge can be accumulated and given to any other
suitable conductor. In course of time, however, the surface charge
of the resin becomes dissipated and it then has to be again excited.
To avoid the necessity for touching the upper plate every time
it is put down on the resin, a metal pin may be brought through
the insulator from the sole-plate so that each time that the
upper plate is put down on the resin it is automatically connected
to earth. We are thus able by a process of merely lifting the
upper plate repeatedly to convey a large electrical charge to
some conductor starting from the small charge produced by
friction on the resin. The above explanation does not take into
account the function of the sole-plate, which is important. The
sole-plate serves to increase the electrical capacity of the upper
plate when placed down upon the resin or excited insulator.
Hence when so placed it takes a larger charge. When touched
by the finger the upper plate is brought to zero potential. If
then the upper plate is lifted by its insulating handle its capacity
becomes diminished. Since, however, it carries with it the charge
it had when resting on the resin, its potential becomes increased
as its capacity becomes less, and it therefore rises to a high
potential, and will give a spark if the knuckle is approached to
it when it is lifted after having been touched and raised.
The study of Volta’s electrophorus at once suggested the performance of these cyclical operations by some form of rotation instead of elevation, and led to the invention of various forms of doubler or multiplier. The instrument was thus the first of a long series of machines for converting mechanical work into electrostatic energy, and the predecessor of the modern type of influence machine (see Electrical Machine). Volta himself devised a double and reciprocal electrophorus and also made mention of the subject of multiplying condensers in a paper published in the Phil. Trans. for 1782 (p. 237, and appendix, p. vii.). He states, however, that the use of a condenser in connexion with an electrophorus to make evident and multiply weak charges was due to T. Cavallo (Phil. Trans., 1788).
For further information see S. P. Thompson, “The Influence Machine from 1788 to 1888,” Journ. Inst. Tel. Eng., 1888, 17, p. 569. Many references to original papers connected with the electrophorus will be found in A. Winkelmann’s Handbuch der Physik (Breslau, 1905), vol. iv. p. 48. (J. A. F.)
ELECTROPLATING, the art of depositing metals by the
electric current. In the article Electrolysis it is shown how
the passage of an electric current through a solution containing
metallic ions involves the deposition of the metal on the cathode.
Sometimes the metal is deposited in a pulverulent form, at others
as a firm tenacious film, the nature of the deposit being dependent
upon the particular metal, the concentration of the solution, the
difference of potential between the electrodes, and other experimental
conditions. As the durability of the electro-deposited
- ↑ See J. A. Fleming, Handbook for the Electrical Laboratory and Testing Room, vol. i. p. 448 (London, 1901).
