[1] It is probable that an experiment of this kind had been made as
far back as 1746 by Daniel Gralath, of Danzig, who has some claims to
have suggested the word "electrometer" in connexion with it. See Park
Benjamin, _The Intellectual Rise in Electricity_ (London, 1895), p.
542.
[2] See Maxwell, _Treatise on Electricity and Magnetism_ (2nd ed.),
i. 308.
[3] See Maxwell, _Electricity and Magnetism_ (2nd ed., Oxford, 1881),
vol. i. p. 311.
[4] See J.A. Fleming, _Handbook for the Electrical Laboratory and
Testing Room_, vol. i. p. 448 (London, 1901).
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 1/2000 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 coat on plated wares of all kinds is of the utmost
importance, the greatest care must be taken to ensure its complete
adhesion. This can only be effected if the surface of the metal on which
the deposit is to be made is chemically clean. Grease must be removed by
potash, whiting or other means, and tarnish by an acid or potassium
cyanide, washing in plenty of water being resorted to after each
operation. The vats for depositing may be of enamelled iron, slate,
glazed earthenware, glass, lead-lined wood, &c. The current densities
and potential differences frequently used for some of the commoner
metals are given in the following table, taken from M'Millan's _Treatise
on Electrometallurgy_. It must be remembered, however, that variations
in conditions modify the electromotive force required for any given
process. For example, a rise in temperature of the bath causes an
increase in its conductivity, so that a lower E.M.F. will suffice to
give the required current density; on the other hand, an abnormally
great distance between the electrodes, or a diminution in acidity of an
acid bath, or in the strength of the solution used, will increase the
resistance, and so require the application of a higher E.M.F.
+----------------------+------------------------------------+---------------+
| | Amperes. | |
| +-------------------+----------------+ Volts between |
| Metal. | Per sq. decimetre | Per sq. in. of | Anode and |
| | of Cathode | Cathode | Cathode. |
| | Surface. | Surface. | |
+----------------------+-------------------+----------------+---------------+
| Antimony | 0.4-0.5 | 0.02-0.03 | 1.0-1.2 |
| Brass | 0.5-0.8 | 0.03-0.05 | 3.0-4.0 |
| Copper, acid bath | 1.0-1.5 | 0.065-0.10 | 0.5-1.5 |
| " alkaline bath| 0.3-0.5 | 0.02-0.03 | 3.0-5.0 |
| Gold | 0.1 | 0.006 | 0.5-4.0 |
| Iron | 0.5 | 0.03 | 1.0 |
| Nickel, at first | 1.4-1.5 | 0.09-0.10 | 5.0 |
| " after | 0.2-0.3 | 0.015-0.02 | 1.5-2.0 |
| " on zinc | 0.4 | 0.025 | 4.0-5.0 |
| Silver | 0.2-0.5 | 0.015-0.03 | 0.75-1.0 |
| Zinc | 0.3-0.6 | 0.02-0.04 | 2.5-3.0 |
+----------------------+-------------------+----------------+---------------+
Large objects are suspended in the tanks by hooks or wires, care being
taken to shift their position and so avoid wire-marks. Small objects are
often heaped together in perforated trays or ladles, the cathode
connecting-rod being buried in the midst of them. These require constant
shifting because the objects are in contact at many points, and because
the top ones shield those below from the depositing action of the
current. Hence processes have been patented in which the objects to be
plated are suspended in revolving drums between the anodes, the rotation
of the drum causing the constant renewal of surfaces and affording a
burnishing action at the same time. Care must be taken not to expose
goods in the plating-bath to too high a current density, else they may
be "burnt"; they must never be exposed one at a time to the full anode
surface, with the current flowing in an empty bath, but either one piece
at a time should be replaced, or some of the anodes should be
transferred temporarily to the place of the cathodes, in order to
distribute the current over a sufficient cathode-area. Burnt deposits
are dark-coloured, or even pulverulent and useless. The strength of the
current may also be regulated by introducing lengths of German silver or
iron wire, carbon rod, or other inferior conductors in the path of the
current, and a series of such resistances should always be provided
close to the tanks. Ammeters to measure the volume, and voltmeters to
determine the pressure of current supplied to the baths, should also be
provided. Very irregular surfaces may require the use of specially
shaped anodes in order that the distance between the electrodes may be
fairly uniform, otherwise the portion of the cathode lying nearest to
the anode may receive an undue share of the current, and therefore a
greater thickness of coat. Supplementary anodes are sometimes used in
difficult cases of this kind. Large metallic surfaces (especially
external surfaces) are sometimes plated by means of a "doctor," which,
in its simplest form, is a brush constantly wetted with the electrolyte,
with a wire anode buried amid the hairs or bristles; this brush is
painted slowly over the surface of the metal to be coated, which must be
connected to the negative terminal of the electrical generator. Under
these conditions electrolysis of the solution in the brush takes place.
Iron ships' plates have recently been coated with copper in sections (to
prevent the adhesion of barnacles), by building up a temporary trough
against the side of the ship, making the thoroughly cleansed plate act
both as cathode and as one side of the trough. Decorative plating-work
in several colours (e.g. "parcel-gilding") is effected by painting a
portion of an object with a stopping-out (i.e. a non-conducting)
varnish, such as copal varnish, so that this portion is not coated. The
varnish is then removed, a different design stopped out, and another
metal deposited. By varying this process, designs in metals of different
colours may readily be obtained.
Reference must be made to the textbooks (see ELECTROCHEMISTRY) for a
fuller account of the very varied solutions and methods employed for
electroplating with silver, gold, copper, iron and nickel. It should be
mentioned here, however, that solutions which would deposit their metal
on any object by simple immersion should not be generally used for
electroplating that object, as the resulting deposit is usually
non-adhesive. For this reason the acid copper-bath is not used for iron
or zinc objects, a bath containing copper cyanide or oxide dissolved in
potassium cyanide being substituted. This solution, being an inferior
conductor of electricity, requires a much higher electromotive force to
drive the current through it, and is therefore more costly in use. It
is, however, commonly employed hot, whereby its resistance is reduced.
_Zinc_ is commonly deposited by electrolysis on iron or steel goods
which would ordinarily be "galvanized," but which for any reason may not
conveniently be treated by the method of immersion in fused zinc. The
zinc cyanide bath may be used for small objects, but for heavy goods the
sulphate bath is employed. Sherard Cowper-Coles patented a process in
which, working with a high current density, a lead anode is used, and
powdered zinc is kept suspended in the solution to maintain the
proportion of zinc in the electrolyte, and so to guard against the
gradual acidification of the bath. _Cobalt_ is deposited by a method
analogous to that used for its sister-metal nickel. _Platinum_,
_palladium_ and _tin_ are occasionally deposited for special purposes.
In the deposition of _gold_ the colour of the deposit is influenced by
the presence of impurities in the solution; when copper is present, some
is deposited with the gold, imparting to it a reddish colour, whilst a
little silver gives it a greenish shade. Thus so-called coloured-gold
deposits may be produced by the judicious introduction of suitable
impurities. Even pure gold, it may be noted, is darker or lighter in
colour according as a stronger or a weaker current is used. The
electro-deposition of _brass_--mainly on iron ware, such as bedstead
tubes--is now very widely practised, the bath employed being a mixture
of copper, zinc and potassium cyanides, the proportions of which vary
according to the character of the brass required, and to the mode of
treatment. The colour depends in part upon the proportion of copper and
zinc, and in part upon the current density, weaker currents tending to
produce a redder or yellower metal. Other alloys may be produced, such
as bronze, or German silver, by selecting solutions (usually cyanides)
from which the current is able to deposit the constituent metals
simultaneously.
Electrolysis has in a few instances been applied to processes of
manufacture. For example, Wilde produced copper printing surfaces for
calico printing-rollers and the like by immersing rotating iron
cylinders as cathodes in a copper bath. Elmore, Dumoulin, Cowper-Coles
and others have prepared copper cylinders and plates by depositing
copper on rotating mandrels with special arrangements. Others have
arranged a means of obtaining high conductivity wire from cathode-copper
without fusion, by depositing the metal in the form of a spiral strip on
a cylinder, the strip being subsequently drawn down in the usual way; at
present, however, the ordinary methods of wire production are found to
be cheaper. J.W. Swan (_Journ. Inst. Elec. Eng._, 1898, vol. xxvii. p.
16) also worked out, but did not proceed with, a process in which a
copper wire whilst receiving a deposit of copper was continuously passed
through the draw-plate, and thus indefinitely extended in length.
Cowper-Coles (_Journ. Inst. Elec. Eng._, 1898, 27, p. 99) very
successfully produced true parabolic reflectors for projectors, by
depositing copper upon carefully ground and polished glass surfaces
rendered conductive by a film of deposited silver.
ELECTROSCOPE, an instrument for detecting differences of electric
potential and hence electrification. The earliest form of scientific
electroscope was the _versorium_ or electrical needle of William Gilbert
(1544-1603), the celebrated author of the treatise _De magnete_ (see
ELECTRICITY). It consisted simply of a light metallic needle balanced on
a pivot like a compass needle. Gilbert employed it to prove that
numerous other bodies besides amber are susceptible of being electrified
by friction.[1] In this case the visible indication consisted in the
attraction exerted between the electrified body and the light pivoted
needle which was acted upon and electrified by induction. The next
improvement was the invention of simple forms of repulsion electroscope.
Two similarly electrified bodies repel each other. Benjamin Franklin
employed the repulsion of two linen threads, C.F. de C. du Fay, J.
Canton, W. Henley and others devised the pith ball, or double straw
electroscope (fig. 1). T. Cavallo about 1770 employed two fine silver
wires terminating in pith balls suspended in a glass vessel having
strips of tin-foil pasted down the sides (fig. 2). The object of the
thimble-shaped dome was to keep moisture from the stem from which the
pith balls were supported, so that the apparatus could be used in the
open air even in the rainy weather. Abraham Bennet (_Phil. Trans._,
1787, 77, p. 26) invented the modern form of gold-leaf electroscope.
Inside a glass shade he fixed to an insulated wire a pair of strips of
gold-leaf (fig. 3). The wire terminated in a plate or knob outside the
vessel. When an electrified body was held near or in contact with the
knob, repulsion of the gold leaves ensued. Volta added the condenser
(_Phil. Trans._, 1782), which greatly increased the power of the
instrument. M. Faraday, however, showed long subsequently that to bestow
upon the indications of such an electroscope definite meaning it was
necessary to place a cylinder of metallic gauze connected to the earth
inside the vessel, or better still, to line the glass shade with
tin-foil connected to the earth and observe through a hole the
indications of the gold leaves (fig. 4). Leaves of aluminium foil may
with advantage be substituted for gold-leaf, and a scale is sometimes
added to indicate the angular divergence of the leaves.
[Illustration: FIG. 1.--Henley's Electroscope.]
[Illustration: FIG. 2.--Cavallo's Electroscope.]
[Illustration: FIG. 3.--Bennet's Electroscope.]
The uses of an electroscope are, first, to ascertain if any body is in a
state of electrification, and secondly, to indicate the sign of that
charge. In connexion with the modern study of radioactivity, the
electroscope has become an instrument of great usefulness, far
outrivalling the spectroscope in sensibility. Radio-active bodies are
chiefly recognized by the power they possess of rendering the air in
their neighbourhood conductive; hence the electroscope detects the
presence of a radioactive body by losing an electric charge given to it
more quickly than it would otherwise do. A third great use of the
electroscope is therefore to detect electric conductivity either in the
air or in any other body.
[Illustration: FIG. 4.--Gold-Leaf Electroscope.]
To detect electrification it is best to charge the electroscope by
induction. If an electrified body is held near the gold-leaf
electroscope the leaves diverge with electricity of the same sign as
that of the body being tested. If, without removing the electrified
body, the plate or knob of the electroscope is touched, the leaves
collapse. If the electroscope is insulated once more and the electrified
body removed, the leaves again diverge with electricity of the opposite
sign to that of the body being tested. The sign of charge is then
determined by holding near the electroscope a glass rod rubbed with silk
or a sealing-wax rod rubbed with flannel. If the approach of the glass
rod causes the leaves in their final state to collapse, then the charge
in the rod was positive, but if it causes them to expand still more the
charge was negative, and vice versa for the sealing-wax rod. When
employing a Volta condensing electroscope, the following is the method
of procedure:--The top of the electroscope consists of a flat, smooth
plate of lacquered brass on which another plate of brass rests,
separated from it by three minute fragments of glass or shellac, or a
film of shellac varnish. If the electrified body is touched against the
upper plate whilst at the same time the lower plate is put to earth, the
condenser formed of the two plates and the film of air or varnish
becomes charged with positive electricity on the one plate and negative
on the other. On insulating the lower plate and raising the upper plate
by the glass handle, the capacity of the condenser formed by the plates
is vastly decreased, but since the charge on the lower plate including
the gold leaves attached to it remains the same, as the capacity of the
system is reduced the potential is raised and therefore the gold leaves
diverge widely. Volta made use of such an electroscope in his celebrated
experiments (1790-1800) to prove that metals placed in contact with one
another are brought to different potentials, in other words to prove the
existence of so-called contact electricity. He was assisted to detect
the small potential differences then in question by the use of a
multiplying condenser or revolving doubler (see ELECTRICAL MACHINE). To
employ the electroscope as a means of detecting radioactivity, we have
first to test the leakage quality of the electroscope itself. Formerly
it was usual to insulate the rod of the electroscope by passing it
through a hole in a cork or mass of sulphur fixed in the top of the
glass vessel within which the gold leaves were suspended. A further
improvement consisted in passing the metal wire to which the gold leaves
were attached through a glass tube much wider than the rod, the latter
being fixed concentrically in the glass tube by means of solid shellac
melted and run in. This insulation, however, is not sufficiently good
for an electroscope intended for the detection of radioactivity; for
this purpose it must be such that the leaves will remain for hours or
days in a state of steady divergence when an electrical charge has been
given to them.
In their researches on radioactivity M. and Mme P. Curie employed an
electroscope made as follows:--A metal case (fig. 5), having two holes
in its sides, has a vertical brass strip B attached to the inside of the
lid by a block of sulphur SS or any other good insulator. Joined to the
strip is a transverse wire terminating at one end in a knob C, and at
the other end in a condenser plate P'. The strip B carries also a strip
of gold-leaf L, and the metal case is connected to earth. If a charge is
given to the electroscope, and if any radioactive material is placed on
a condenser plate P attached to the outer case, then this substance
bestows conductivity on the air between the plates P and P', and the
charge of the electroscope begins to leak away. The collapse of the
gold-leaf is observed through an aperture in the case by a microscope,
and the time taken by the gold-leaf to fall over a certain distance is
proportional to the ionizing current, that is, to the intensity of the
radioactivity of the substance.
[Illustration: FIG. 5.--Curie's Electroscope.]
A very similar form of electroscope was employed by J.P.L.J. Elster and
H.F.K. Geitel (fig. 6), and also by C.T.R. Wilson (see _Proc. Roy.
Soc._, 1901, 68, p. 152). A metal box has a metal strip B suspended from
a block or insulator by means of a bit of sulphur or amber S, and to it
is fastened a strip of gold-leaf L. The electroscope is provided with a
charging rod C. In a dry atmosphere sulphur or amber is an early perfect
insulator, and hence if the air in the interior of the box is kept dry
by calcium chloride, the electroscope will hold its charge for a long
time. Any divergence or collapse of the gold-leaf can be viewed by a
microscope through an aperture in the side of the case.
[Illustration: FIG. 6.--Elster and Geitel Electroscope.]
[Illustration: FIG. 7.--Wilson's Electroscope.]
Another type of sensitive electroscope is one devised by C.T.R. Wilson
(_Proc. Cam. Phil. Soc._, 1903, 12, part 2). It consists of a metal box
placed on a tilting stand (fig. 7). At one end is an insulated plate P
kept at a potential of 200 volts or so above the earth by a battery. At
the other end is an insulated metal wire having attached to it a thin
strip of gold-leaf L. If the plate P is electrified it attracts the
strip which stretches out towards it. Before use the strip is for one
moment connected to the case, and the arrangement is then tilted until
the strip extends at a certain angle. If then the strip of gold-leaf is
raised or lowered in potential it moves to or from the plate P, and its
movement can be observed by a microscope through a hole in the side of
the box. There is a particular angle of tilt of the case which gives a
maximum sensitiveness. Wilson found that with the plate electrified to
207 volts and with a tilt of the case of 30 deg., if the gold-leaf was
raised one volt in potential above the case, it moved over 200 divisions
of the micrometer scale in the eye-piece of the microscope, 54 divisions
being equal to one millimetre. In using the instrument the insulated rod
to which the gold-leaf is attached is connected to the conductor, the
potential of which is being examined. In the use of all these
electroscopic instruments it is essential to bear in mind (as first
pointed out by Lord Kelvin) that what a gold-leaf electroscope really
indicates is the difference of potential between the gold-leaf and the
solid walls enclosing the air space in which they move.[2] If these
enclosing walls are made of anything else than perfectly conducting
material, then the indications of the instrument may be uncertain and
meaningless. As already mentioned, Faraday remedied this defect by
coating the inside of the glass vessel in which the gold-leaves were
suspended to form an electroscope with tinfoil (see fig. 4). In spite of
these admonitions all but a few instrument makers have continued to make
the vicious type of instrument consisting of a pair of gold-leaves
suspended within a glass shade or bottle, no means being provided for
keeping the walls of the vessel continually at zero potential.
See J. Clerk Maxwell, _Treatise on Electricity and Magnetism_, vol. i.
p. 300 (2nd ed., Oxford, 1881); H.M. Noad, _A Manual of Electricity_,
vol. i. p. 25 (London, 1855); E. Rutherford, _Radioactivity_.
(J. A. F.)