THEORIES OF ELECTRICITY
The science of electricity is based upon observation of those phenomena
of attraction and repulsion which are comprehended under the term
_electrostatics_. Statical electricity, so named from a Greek
word (statikos), which means “causing to stand (or stay),”—also called
_frictional electricity_—is the electricity of stationary charges
caused by rubbing together unlike bodies, such as glass and silk (noted
in Chapter II). In such cases equal and opposite charges of electricity
are always produced. The term _statical electricity_ applies
properly, however, to the electricity of all stationary charges,
however produced.
The electricity upon the surface of glass is called positive
electricity; that upon rubber, negative electricity. When silk is
rubbed upon glass it receives a negative charge from the glass and
confers a positive charge upon the silk. Wool or fur rubbed on wax or
rubber receives a positive charge in exchange for a negative charge;
“equal and opposite charges of electricity are always produced.” A
piece of glass and a piece of silk attract one another; two pieces of
silk or two pieces of glass or wax repel one another, because a body
which is positively charged is attracted by one negatively charged
and repelled by one negatively charged, and vice versa. A piece of
glass rubbed by a piece of silk, under suitable conditions, attracts
any other body with which it has not been in contact. The piece of
silk will do likewise. In all these cases, the attraction or repulsion
becomes weaker with increase of distance between the attracting and
repelling bodies.
A third body which has been in contact with a piece of glass or a piece
of silk acquires to some extent the properties of the glass or silk
with which the third body has been in contact. And, conversely, the
glass or silk with which the third body has been in contact attracts
or repels with less force than before. If a hand is drawn over the
surface of an object after it has been charged with electricity, the
electricity disappears. It has been conducted through the hand and
the body to the earth. This phenomenon shows that the human body is
a _conductor_ of electricity. But most metals are much better
conductors. Moist air and damp wood are rather poor conductors, while
dry air, dry wood, porcelain, glass, hard rubber and sealing-wax are
_non-conductors_, or _insulators_.
The term _dielectric_ is used in preference to _insulation_
when reference is made to the property of transmitting
_induction_—a process quite distinct from ordinary transmission
of an electric current. In _electrostatic induction_, a body
electrostatically charged induces in a neighboring conductor a like
charge in the parts farthest from the charged body, and an unlike
charge in the nearer parts; the repelled like charge being removed by
connecting any part of the conductor momentarily with the earth, while
the bound unlike charge spreads over the whole surface of the conductor
and remains there even when the inducing body is moved away, or its
charge neutralized, if the conductor is properly insulated.
_Dielectric strength_ refers to the ability of an insulating
material to resist rupture by high voltage, measured by the voltage
necessary to effect a disruptive discharge through it. _Insulation
resistance_, on the other hand, refers to the _ohmic_
resistance _offered_ by an insulating material to an impressed
voltage, tending to induce a breakage of current through it. The term
_dielectric_ is used as a synonym for _insulator_, in the
sense that a charge on one part of a non-conductor is not communicated
to any other part. A charge given to a conductor spreads to all parts
of the body. A dielectric possesses the property of transmitting
electric force by _in_duction but not by _con_duction. A
charge on one part of a non-conductor or dielectric is not communicated
to any other part.
Jeans suggests that since the presence of magnetic energy is always
associated with charges in motion, whereas electrostatic energy is
present when all the charges are at rest relatively to each other, it
may be proper to identify electrostatic energy with potential energy,
and magnetic energy with kinetic energy[14]—i. e., energy due to motion
of particles, rather than to energy of position, as of a coiled spring.
Statical energy is distinguished from “current electricity” by the
fact that it accumulates on various bodies—is stored up—and as soon
as proper connections are made, it discharges instantly. Statical
electricity is used by physicians in electrical treatment of diseases
and in X-ray work. Machines have been constructed that will produce
very strong charges of statical electricity.
If a sufficiently large charge of electricity accumulates upon an
insulated conductor in an electrical machine, it finally discharges
itself, passing through the air to the nearest body. A flash of
lightning is the result of an overcharge of statical electricity
accumulated upon cloud particles, and may pass from cloud to cloud
or descend to the earth.[15] Careful drivers of gasoline-tank wagons
allow an iron or steel chain to drag on the roadway from a metallic
connection, which conducts any surplus “static” to the ground. Failure
to provide for such an emergency sometimes results in a terrific
explosion with consequent loss of life.
About the beginning of the nineteenth century, the Italian scientist,
Alessandro Volta (1745-1827),—and other physicists—discovered what
has been called, after Volta, _voltaic electricity_, a current
generated by chemical action between metals and different liquids as
arranged in a voltaic battery. The term “volt”—the electromotive force
which performs work at the rate of one joule per second (one watt) in
producing a current of one ampere—was similarly derived.
It was learned that if two different metals, such as copper and
zinc amalgam, are placed in a weak acid solution (such as one part
H_{2}SO_{4} to four parts H_{2}O), and connected by a wire fastened
securely to the metals, a current of electricity (about two volts) will
pass through the wire. Carbon (a non-metal) and a metal upon which the
solution acts chemically may be used instead of two metals. There must
be chemical action between the liquid and one metal, or there will be
no current. Such a combination constitutes a _cell_, and two or
more cells make a _battery_. The current starts with the zinc,
is conducted by the solution to the copper, and thence by wire back
to the zinc, completing a _circuit_. The zinc constitutes the
negative pole (or electrode), the copper or carbon the positive pole
(or electrode).
A cell frequently employed, where a weak (about 1.1 volts) but constant
electromotive force (“E. M. F.”) is required, is one devised by the
English physicist, John D. Daniell (1790-1845). In this cell a copper
sulphate solution containing a copper electrode is placed in contact
(by means of a porous wall or partition—usually an unglazed porcelain
cup) with a zinc sulphate solution containing a zinc electrode. The
zinc electrode is negative to the copper. At each electrode there
exists a potential difference between solution and electrode.[16] The
two electrodes being connected externally by a wire, a current of
electricity will flow through the wire from the copper to the zinc, and
zinc will dissolve at the anode (positive pole) and copper deposited
on the cathode (negative pole). The current in this case, as in the
preceding, is said to be produced by _voltaic action_ and the
cell is a primary battery. Voltaic action and _electrolysis_—the
process of chemical decomposition (or dissociation of compounds or
molecules)—by the action of an electric current produced externally (as
by a dynamo) and forced through the cell, are essentially identical
phenomena, and obey the same laws.[17]
The familiar _dry cell_ contains no liquid which might be spilled,
and is very useful for certain purposes, as in automobiles, and in
operating door-bells. It is merely a voltaic cell whose chemical
contents are made practically solid (or paste-like) by the use of some
absorbent, as gelatine, sawdust, etc. In cells of the Leclanché type,
a mixture of plaster of Paris, flour, and sal ammoniac takes the place
of the solution which acts chemically upon one of the contained metals.
When used up, a dry cell must be replaced by an entirely new cell. Two
or more dry cells constitute a _dry battery_.
We have seen that there are two types of charged bodies, of which
charged glass and charged silk are familiar examples. It was Dufay
(1699-1739) who discovered that there were two kinds of electricity,
one of which he called _vitreous_ (from glass) and the other
_resinous_ (from resin—amber). The terms “positive” and “negative”
in relation to electricity were first applied by Benjamin Franklin,
in 1756. To the electricity of the glass rod Franklin gave the name
“positive” and to that of the sealing-wax (or hard rubber, amber, etc.)
the name “negative.” These names are now universally in use—though
French physicists still speak of vitreous and resinous electricity.
I have spoken also of a positive pole (or electrode) and a negative
pole (or electrode). The electrodes constituting the two poles of a
current are also called the anode and the cathode, the former being the
positive electrode and the latter the negative electrode.[18]
When it was learned that electrical charges could be distinguished by
two opposing terms—positive and negative—it was natural to suppose that
there were two distinct kinds of electricity, or “fluids.” This was
the view taken by the French chemist Dufay. But the German electrician
Æpinus (1724-1802), in his great pioneer work, “_Tentamen Theoriae
Electriciatis et Magnetismi_” (An Attempt at a Theory of Electricity
and Magnetism—1759), considered the mathematical consequences of the
hypothesis of a single fluid, attracting all matter but repelling
itself. It soon became apparent, however, that he must assume either
the existence of two electrical fluids or the mutual repulsion of
material particles. He chose the latter theory. He explained the
phenomena of the opposite poles as results of the excess and deficiency
of a “magnetic fluid,” which was dislodged and accumulated in the
ends of the body, by the repulsion of its own particles, and by the
attraction of iron and steel, as in the case of induced electricity.[19]
Æpinus, who was unquestionably one of the greatest physicists of the
eighteenth century, devised a method of examining the nature of the
electricity at any part of the surface of a body, by which means he was
enabled to ascertain its distribution. He found that the distribution
was in agreement with the attractions and repulsions which objects
exert when they are in the neighborhood—“electrical atmosphere”—of
electrified bodies. Today we say that such bodies are electrified by
induction.
The Æpinian theory of electricity and of magnetism was modified and
presented in a new form (in 1788) by Coulomb, with two fluids instead
of one. His first task, before reducing the theory to calculation,
was to determine the law of the forces involved—not being satisfied,
for example, with Newton’s assumption that the attractive force of
magnetism is inversely to the _cube_ of the distance. Mayer in
1760, and Lambert a few years later, had found the law to be that of
the inverse square. Coulomb desired experimental confirmation of this
law before accepting it as established. This he secured by means of
his torsion-balance (about 1784).[20]
It was in pursuance of this investigation that Coulomb brought to light
for the first time the fact that the directive magnetic forces which
the earth exerts upon a needle is a constant quantity, parallel to the
magnetic meridian, and passing through the same point of the needle
whatever be its position.
Barlow, who had adopted the two-fluid hypothesis, showed that the
magnetic “fluids” were collected at the surface of spheres (of iron),
the surface being the only part in which there could be detected any
magnetism. He demonstrated that a shell of iron produces the same
effect as a solid ball of the same diameter. Poisson’s later analysis
(1824) showed that this was a consequent to be expected. Merz has well
said that what Laplace did for Newton was done by Poisson (1781-1840)
“for Coulomb’s elementary law of electric and magnetic action, and
on a still larger scale by Gauss, who worked out the mathematical
theory and applied it to the case of the magnetic distribution on the
earth’s surface. In England, already before Coulomb’s researches were
published, Cavendish had, likewise by a combination of experiment and
calculation, established the elementary formulae and properties of
electrical phenomena.”[21]
Benjamin Franklin, the first American to gain international renown as a
scientist, adopted and developed a “one-fluid theory of electricity.”
On this supposition the parts of the fluid repel each other, and
the excess in one surface of the glass—for example—repels the fluid
from the other surface. The fluid itself was regarded by Franklin as
positive, the part of the other (negative electricity) being taken by
ordinary matter, the particles of which were supposed to repel each
other and attract the positive fluid, just as the particles of the
negative fluid did on the two-fluid theory.
On both the two-fluid and the one-fluid theories, as we have seen, the
particles of the positive fluid repelled each other by forces varying
inversely as the square of the distance between them—as shown by both
Æpinus and Coulomb. This is true also of the particles of the negative
fluid. The particles of the positive fluid attracted those of the
negative fluid. In Franklin’s one-fluid theory it was the ordinary
particles of matter which attracted the positive fluid and repelled
one another. Both theories from their very nature imply, as Sir J. J.
Thomson long ago (1906) pointed out, the idea of action at a distance.
In his very interesting book, “Matter and Energy” (1912), Professor
Soddy says: “All electrical phenomena can be explained as well on the
one-fluid as on the two-fluid idea, but our ignorance at the present
time as to whether there are two kinds of electricity or one is
fundamental. Until the question is settled, the hopes that have been
entertained that, through the study of electricity, we shall be able to
arrive at a philosophical explanation of matter, are likely to prove
unfounded.”
Our modern view of electrification bears a close resemblance to the
one-fluid theory of Franklin, whether we suppose there is one kind of
electricity, or two kinds. At all events, if there be such a separate
force, or such units of energy, as “positive” electricity, it has never
been isolated, as have been the negative atoms or electrons. Negative
electrification is but a collection of these negative corpuscles
or unit charges. The particles of the “electric fluid” of Franklin
correspond to these electrons.
“Instead of taking, as Franklin did, the electric fluid to be positive
electricity, we take it to be negative,” says J. J. Thomson, in
his “Corpuscular Theory of Matter” (1906). And “the transference
of electrification from one place to another is effected by this
motion of corpuscles from the place where there is a gain of positive
electrification to the place where there is a gain of negative.
A positively electrified body is one that has lost some of its
corpuscles.”[22]
FOOTNOTES:
[14] Jeans, J. H., “Electricity and Magnetism,” Page 483, 1911.
[15] Benjamin Franklin was first to show (in a letter to Peter
Collinson, written October 19, 1752) that lightning and electricity are
one and the same thing. He was also inventor of the lightning-rod.
[16] “Potential” is analogous to level (or pressure) in hydrostatics or
mechanics.
[17] For further explanation, see Shipley, Maynard, “The A. B. C. of
the Electronic Theory of Matter,” Little Blue Book Series, No. 603.
[18] See, in this connection, Shipley, _Op. cit._
[19] A very similar hypothesis was read before the Royal Society by
Henry Cavendish, in 1771, the work of Æpinus being unknown to him at
the time.
[20] By means of this instrument very minute forces can be accurately
measured, such as electrostatic or magnetic attraction and repulsion,
by the torsion (turning or twisting) of a wire or filament, the angle
of torsion being proportional to the amount of force exerted.
[21] Merz, Henry, “History of European Thought in the Nineteenth
Century,” Vol. I, Page 362.
[22] For a recent work on modern electrical theory, see Starling,
Sydney G., (head of the department of physics in the West Ham Municipal
College, London), “Electricity,” London, 1922. For the pioneer work of
Ampère, see his “_Theorie des Phenomenes Electrodynamiques_,” 1826.