FOOTNOTE:

[1] See Lord Kelvin, _Reprint of Papers on Electrostatics and

Magnetism_ (1872); "Electrophoric Apparatus and Illustrations of

Voltaic Theory," p. 319; "On Electric Machines Founded on Induction

and Convection," p. 330; "The Reciprocal Electrophorus," p. 337.

ELECTRIC EEL (_Gymnotus electricus_), a member of the family of fishes

known as _Gymnotidae_. In spite of their external similarity the

_Gymnotidae_ have nothing to do with the eels (_Anguilla_). They

resemble the latter in the elongation of the body, the large number of

vertebrae (240 in _Gymnotus_), and the absence of pelvic fins; but they

differ in all the more important characters of internal structure. They

are in fact allied to the carps or _Cyprinidae_ and the cat-fishes or

_Siluridae_. In common with these two families and the _Characinidae_ of

Africa and South America, the _Gymnotidae_ possess the peculiar

structures called _ossicula auditus_ or Weberian ossicles. These are a

chain of small bones belonging to the first four vertebrae, which are

much modified, and connecting the air-bladder with the auditory organs.

Such an agreement in the structure of so complicated and specialized an

apparatus can only be the result of a community of descent of the

families possessing it. Accordingly these families are now placed

together in a distinct sub-order, the Ostariophysi. The _Gymnotidae_ are

strongly modified and degraded _Characinidae_. In them the dorsal and

caudal fins are very rudimentary or absent, and the anal is very long,

extending from the anus, which is under the head or throat, to the end

of the body.

_Gymnotus_ is the only genus of the family which possesses electric

organs. These extend the whole length of the tail, which is four-fifths

of the body. They are modifications of the lateral muscles and are

supplied with numerous branches of the spinal nerves. They consist of

longitudinal columns, each composed of an immense number of "electric

plates." The posterior end of the organ is positive, the anterior

negative, and the current passes from the tail to the head. The maximum

shock is given when the head and tail of the _Gymnotus_ are in contact

with different points in the surface of some other animal. _Gymnotus

electricus_ attains a length of 3 ft. and the thickness of a man's

thigh, and frequents the marshes of Brazil and the Guianas, where it is

regarded with terror, owing to the formidable electrical apparatus with

which it is provided. When this natural battery is discharged in a

favourable position, it is sufficiently powerful to stun the largest

animal; and according to A. von Humboldt, it has been found necessary to

change the line of certain roads passing through the pools frequented by

the electric eels. These fish are eaten by the Indians, who, before

attempting to capture them, seek to exhaust their electrical power by

driving horses into the ponds. By repeated discharges upon these they

gradually expend this marvellous force; after which, being defenceless,

they become timid, and approach the edge for shelter, when they fall an

easy prey to the harpoon. It is only after long rest and abundance of

food that the fish is able to resume the use of its subtle weapon.

Humboldt's description of this method of capturing the fish has not,

however, been verified by recent travellers.

ELECTRICITY. This article is devoted to a general sketch of the history

of the development of electrical knowledge on both the theoretical and

the practical sides. The two great branches of electrical theory which

concern the phenomena of electricity at rest, or "frictional" or

"static" electricity, and of electricity in motion, or electric

currents, are treated in two separate articles, ELECTROSTATICS and

ELECTROKINETICS. The phenomena attendant on the passage of electricity

through solids, through liquids and through gases, are described in the

article CONDUCTION, ELECTRIC, and also ELECTROLYSIS, and the propagation

of electrical vibrations in ELECTRIC WAVES. The interconnexion of

magnetism (which has an article to itself) and electricity is discussed

in ELECTROMAGNETISM, and these manifestations in nature in ATMOSPHERIC

ELECTRICITY; AURORA POLARIS and MAGNETISM, TERRESTRIAL. The general

principles of electrical engineering will be found in ELECTRICITY

SUPPLY, and further details respecting the generation and use of

electrical power are given in such articles as DYNAMO; MOTORS, ELECTRIC;

TRANSFORMERS; ACCUMULATOR; POWER TRANSMISSION: _Electric_; TRACTION;

LIGHTING: _Electric_; ELECTROCHEMISTRY and ELECTROMETALLURGY. The

principles of telegraphy (land, submarine and wireless) and of telephony

are discussed in the articles TELEGRAPH and TELEPHONE, and various

electrical instruments are treated in separate articles such as

AMPEREMETER; ELECTROMETER; GALVANOMETER; VOLTMETER; WHEATSTONE'S BRIDGE;

POTENTIOMETER; METER, ELECTRIC; ELECTROPHORUS; LEYDEN JAR; &c.

The term "electricity" is applied to denote the physical agency which

exhibits itself by effects of attraction and repulsion when particular

substances are rubbed or heated, also in certain chemical and

physiological actions and in connexion with moving magnets and metallic

circuits. The name is derived from the word _electrica_, first used by

William Gilbert (1544-1603) in his epoch-making treatise _De magnete,

magneticisque corporibus, et de magno magnete tellure_, published in

1600,[1] to denote substances which possess a similar property to amber

(= _electrum_, from [Greek: elektron]) of attracting light objects when

rubbed. Hence the phenomena came to be collectively called electrical, a

term first used by William Barlowe, archdeacon of Salisbury, in 1618,

and the study of them, electrical science.

_Historical Sketch._

Gilbert was the first to conduct systematic scientific experiments on

electrical phenomena. Prior to his date the scanty knowledge possessed

by the ancients and enjoyed in the middle ages began and ended with

facts said to have been familiar to Thales of Miletus (600 B.C.) and

mentioned by Theophrastus (321 B.C.) and Pliny (A.D. 70), namely, that

amber, jet and one or two other substances possessed the power, when

rubbed, of attracting fragments of straw, leaves or feathers. Starting

with careful and accurate observations on facts concerning the

mysterious properties of amber and the lodestone, Gilbert laid the

foundations of modern electric and magnetic science on the true

experimental and inductive basis. The subsequent history of electricity

may be divided into four well-marked periods. The first extends from the

date of publication of Gilbert's great treatise in 1600 to the invention

by Volta of the voltaic pile and the first production of the electric

current in 1799. The second dates from Volta's discovery to the

discovery by Faraday in 1831 of the induction of electric currents and

the creation of currents by the motion of conductors in magnetic fields,

which initiated the era of modern electrotechnics. The third covers the

period between 1831 and Clerk Maxwell's enunciation of the

electromagnetic theory of light in 1865 and the invention of the

self-exciting dynamo, which marks another great epoch in the development

of the subject; and the fourth comprises the modern development of

electric theory and of absolute quantitative measurements, and above

all, of the applications of this knowledge in electrical engineering. We

shall sketch briefly the historical progress during these various

stages, and also the growth of electrical theories of electricity during

that time.

FIRST PERIOD.--Gilbert was probably led to study the phenomena of the

attraction of iron by the lodestone in consequence of his conversion to

the Copernican theory of the earth's motion, and thence proceeded to

study the attractions produced by amber. An account of his electrical

discoveries is given in the _De magnete_, lib. ii. cap. 2.[2] He

invented the _versorium_ or electrical needle and proved that

innumerable bodies he called _electrica_, when rubbed, can attract the

needle of the versorium (see ELECTROSCOPE). Robert Boyle added many new

facts and gave an account of them in his book, _The Origin of

Electricity_. He showed that the attraction between the rubbed body and

the test object is mutual. Otto von Guericke (1602-1686) constructed the

first electrical machine with a revolving ball of sulphur (see

ELECTRICAL MACHINE), and noticed that light objects were repelled after

being attracted by excited electrics. Sir Isaac Newton substituted a

ball of glass for sulphur in the electrical machine and made other not

unimportant additions to electrical knowledge. Francis Hawksbee (d.

1713) published in his book _Physico-Mechanical Experiments_ (1709), and

in several Memoirs in the _Phil. Trans._ about 1707, the results of his

electrical inquiries. He showed that light was produced when mercury was

shaken up in a glass tube exhausted of its air. Dr Wall observed the

spark and crackling sound when warm amber was rubbed, and compared them

with thunder and lightning (_Phil. Trans._, 1708, 26, p. 69). Stephen

Gray (1696-1736) noticed in 1720 that electricity could be excited by

the friction of hair, silk, wool, paper and other bodies. In 1729 Gray

made the important discovery that some bodies were conductors and others

non-conductors of electricity. In conjunction with his friend Granville

Wheeler (d. 1770), he conveyed the electricity from rubbed glass, a

distance of 886 ft., along a string supported on silk threads (_Phil.

Trans._, 1735-1736, 39, pp. 16, 166 and 400). Jean Theophile Desaguliers

(1683-1744) announced soon after that electrics were non-conductors, and

conductors were non-electrics. C.F. de C. du Fay (1699-1739) made the

great discovery that electricity is of two kinds, vitreous and resinous

(_Phil. Trans._, 1733, 38, p. 263), the first being produced when glass,

crystal, &c. are rubbed with silk, and the second when resin, amber,

silk or paper, &c. are excited by friction with flannel. He also

discovered that a body charged with positive or negative electricity

repels a body free to move when the latter is charged with electricity

of like sign, but attracts it if it is charged with electricity of

opposite sign, i.e. positive repels positive and negative repels

negative, but positive attracts negative. It is to du Fay also that we

owe the abolition of the distinction between electrics and

non-electrics. He showed that all substances could be electrified by

friction, but that to electrify conductors they must be insulated or

supported on non-conductors. Various improvements were made in the

electrical machine, and thereby experimentalists were provided with the

means of generating strong electrification; C.F. Ludolff (1707-1763) of

Berlin in 1744 succeeded in igniting ether with the electric spark

(_Phil. Trans._, 1744, 43, p. 167).

For a very full list of the papers and works of these early electrical

philosophers, the reader is referred to the bibliography on

Electricity in Dr Thomas Young's _Natural Philosophy_, vol. ii. p.

415.

In 1745 the important invention of the Leyden jar or condenser was made

by E.G. von Kleist of Kammin, and almost simultaneously by Cunaeus and

Pieter van Musschenbroek (1692-1761) of Leiden (see LEYDEN JAR). Sir

William Watson (1715-1787) in England first observed the flash of light

when a Leyden jar is discharged, and he and Dr John Bevis (1695-1771)

suggested coating the jar inside and outside with tinfoil. Watson

carried out elaborate experiments to discover how far the electric

discharge of the jar could be conveyed along metallic wires and was able

to accomplish it for a distance of 2 m., making the important

observation that the electricity appeared to be transmitted

instantaneously.

_Franklin's Researches._--Benjamin Franklin (1706-1790) was one of the

great pioneers of electrical science, and made the ever-memorable

experimental identification of lightning and electric spark. He argued

that electricity is not created by friction, but merely collected from

its state of diffusion through other matter by which it is attracted. He

asserted that the glass globe, when rubbed, attracted the electrical

fire, and took it from the rubber, the same globe being disposed, when

the friction ceases, to give out its electricity to any body which has

less. In the case of the charged Leyden jar, he asserted that the inner

coating of tinfoil had received more than its ordinary quantity of

electricity, and was therefore electrified positively, or plus, while

the outer coating of tinfoil having had its ordinary quantity of

electricity diminished, was electrified negatively, or minus. Hence the

cause of the shock and spark when the jar is discharged, or when the

superabundant or plus electricity of the inside is transferred by a

conducting body to the defective or minus electricity of the outside.

This theory of the Leyden phial Franklin supported very ingeniously by

showing that the outside and the inside coating possessed electricities

of opposite sign, and that, in charging it, exactly as much electricity

is added on one side as is subtracted from the other. The abundant

discharge of electricity by points was observed by Franklin is his

earliest experiments, and also the power of points to conduct it

copiously from an electrified body. Hence he was furnished with a simple

method of collecting electricity from other bodies, and he was enabled

to perform those remarkable experiments which are chiefly connected with

his name. Hawksbee, Wall and J.A. Nollet (1700-1770) had successively

suggested the identity of lightning and the electric spark, and of

thunder and the snap of the spark. Previously to the year 1750, Franklin

drew up a statement, in which he showed that all the general phenomena

and effects which were produced by electricity had their counterparts in

lightning. After waiting some time for the erection of a spire at

Philadelphia, by means of which he hoped to bring down the electricity

of a thunderstorm, he conceived the idea of sending up a kite among

thunder-clouds. With this view he made a small cross of two small light

strips of cedar, the arms being sufficiently long to reach to the four

corners of a large thin silk handkerchief when extended. The corners of

the handkerchief were tied to the extremities of the cross, and when the

body of the kite was thus formed, a tail, loop and string were added to

it. The body was made of silk to enable it to bear the violence and wet

of a thunderstorm. A very sharp pointed wire was fixed at the top of the

upright stick of the cross, so as to rise a foot or more above the wood.

A silk ribbon was tied to the end of the twine next the hand, and a key

suspended at the junction of the twine and silk. In company with his

son, Franklin raised the kite like a common one, in the first

thunderstorm, which happened in the month of June 1752. To keep the silk

ribbon dry, he stood within a door, taking care that the twine did not

touch the frame of the door; and when the thunder-clouds came over the

kite he watched the state of the string. A cloud passed without any

electrical indications, and he began to despair of success. At last,

however, he saw the loose filaments of the twine standing out every way,

and he found them to be attracted by the approach of his finger. The

suspended key gave a spark on the application of his knuckle, and when

the string had become wet with the rain the electricity became abundant.

A Leyden jar was charged at the key, and by the electric fire thus

obtained spirits were inflamed, and many other experiments performed

which had been formerly made by excited electrics. In subsequent trials

with another apparatus, he found that the clouds were sometimes

positively and sometimes negatively electrified, and so demonstrated the

perfect identity of lightning and electricity. Having thus succeeded in

drawing the electric fire from the clouds, Franklin conceived the idea

of protecting buildings from lightning by erecting on their highest

parts pointed iron wires or conductors communicating with the ground.

The electricity of a hovering or a passing cloud would thus be carried

off slowly and silently; and if the cloud was highly charged, the

lightning would strike in preference the elevated conductors.[3] The

most important of Franklin's electrical writings are his _Experiments

and Observations on Electricity made at Philadelphia_, 1751-1754; his

_Letters on Electricity_; and various memoirs and letters in the _Phil.

Trans._ from 1756 to 1760.

About the same time that Franklin was making his kite experiment in

America, T.F. Dalibard (1703-1779) and others in France had erected a

long iron rod at Marli, and obtained results agreeing with those of

Franklin. Similar investigations were pursued by many others, among whom

Father G.B. Beccaria (1716-1781) deserves especial mention. John Canton

(1718-1772) made the important contribution to knowledge that

electricity of either sign could be produced on nearly any body by

friction with appropriate substances, and that a rod of glass roughened

on one half was excited negatively in the rough part and positively in

the smooth part by friction with the same rubber. Canton first suggested

the use of an amalgam of mercury and tin for use with glass cylinder

electrical machines to improve their action. His most important

discovery, however, was that of electrostatic induction, the fact that

one electrified body can produce charges of electricity upon another

insulated body, and that when this last is touched it is left

electrified with a charge of opposite sign to that of the inducing

charge (_Phil. Trans._, 1753-1754). We shall make mention lower down of

Canton's contributions to electrical theory. Robert Symmer (d. 1763)

showed that quite small differences determined the sign of the

electrification that was generated by the friction of two bodies one

against the other. Thus wearing a black and a white silk stocking one

over the other, he found they were electrified oppositely when rubbed

and drawn off, and that such a rubbed silk stocking when deposited in a

Leyden jar gave up its electrification to the jar (_Phil. Trans._,

1759). Ebenezer Kinnersley (1711-1778) of Philadelphia made useful

observations on the elongation and fusion of iron wires by electrical

discharges (_Phil. Trans._, 1763). A contemporary of Canton and

co-discoverer with him of the facts of electrostatic induction was the

Swede, Johann Karl Wilcke (1732-1796), then resident in Germany, who in

1762 published an account of experiments in which a metal plate held

above the upper surface of a glass table was subjected to the action of

a charge on an electrified metal plate held below the glass (_Kon.

Schwedische Akad. Abhandl._, 1762, 24, p. 213).

_Pyro-electricity._--The subject of pyro-electricity, or the power

possessed by some minerals of becoming electrified when merely heated,

and of exhibiting positive and negative electricity, now began to

attract notice. It is possible that the _lyncurium_ of the ancients,

which according to Theophrastus attracted light bodies, was tourmaline,

a mineral found in Ceylon, which had been christened by the Dutch with

the name of _aschentrikker_, or the attractor of ashes. In 1717 Louis

Lemery exhibited to the Paris Academy of Sciences a stone from Ceylon

which attracted light bodies; and Linnaeus in mentioning his experiments

gives the stone the name of _lapis electricus_. Giovanni Caraffa, duca

di Noja (1715-1768), was led in 1758 to purchase some of the stones

called tourmaline in Holland, and, assisted by L.J.M. Daubenton and

Michel Adanson, he made a series of experiments with them, a description

of which he gave in a letter to G.L.L. Buffon in 1759. The subject,

however, had already engaged the attention of the German philosopher,

F.U.T. Aepinus, who published an account of them in 1756. Hitherto

nothing had been said respecting the necessity of heat to excite the

tourmaline; but it was shown by Aepinus that a temperature between

99-1/2 deg. and 212 deg. Fahr. was requisite for the development of its

attractive powers. Benjamin Wilson (_Phil. Trans._, 1763, &c.), J.

Priestley, and Canton continued the investigation, but it was reserved

for the Abbe Hauy to throw a clear light on this curious branch of the

science (_Traite de mineralogie_, 1801). He found that the electricity

of the tourmaline decreased rapidly from the summits or poles towards

the middle of the crystal, where it was imperceptible; and he discovered

that if a tourmaline is broken into any number of fragments, each

fragment, when excited, has two opposite poles. Hauy discovered the same

property in the Siberian and Brazilian topaz, borate of magnesia,

mesotype, prehnite, sphene and calamine. He also found that the polarity

which minerals receive from heat has a relation to the secondary forms

of their crystals--the tourmaline, for example, having its resinous pole

at the summit of the crystal which has three faces. In the other

pyro-electric crystals above mentioned, Hauy detected the same deviation

from the rules of symmetry in their secondary crystals which occurs in

tourmaline. C.P. Brard (1788-1838) discovered that pyro-electricity was

a property of axinite; and it was afterwards detected in other minerals.

In repeating and extending the experiments of Hauy much later, Sir David

Brewster discovered that various artificial salts were pyro-electric,

and he mentions the tartrates of potash and soda and tartaric acid as

exhibiting this property in a very strong degree. He also made many

experiments with the tourmaline when cut into thin slices, and reduced

to the finest powder, in which state each particle preserved its

pyro-electricity; and he showed that scolezite and mesolite, even when

deprived of their water of crystallization and reduced to powder, retain

their property of becoming electrical by heat. When this white powder is

heated and stirred about by any substance whatever, it collects in

masses like new-fallen snow, and adheres to the body with which it is

stirred.

For Sir David Brewster's work on pyro-electricity, see _Trans. Roy.

Soc. Edin._, 1845, also _Phil. Mag._, Dec. 1847. The reader will also

find a full discussion on the subject in the _Treatise on

Electricity_, by A. de la Rive, translated by C.V. Walker (London,

1856), vol. ii. part v. ch. i.

_Animal electricity._--The observation that certain animals could give

shocks resembling the shock of a Leyden jar induced a closer examination

of these powers. The ancients were acquainted with the benumbing power

of the torpedo-fish, but it was not till 1676 that modern naturalists

had their attention again drawn to the fact. E. Bancroft was the first

person who distinctly suspected that the effects of the torpedo were

electrical. In 1773 John Walsh (d. 1795) and Jan Ingenhousz (1730-1799)

proved by many curious experiments that the shock of the torpedo was an

electrical one (_Phil. Trans._, 1773-1775); and John Hunter (id. 1773,

1775) examined and described the anatomical structure of its electrical

organs. A. von Humboldt and Gay-Lussac (_Ann. Chim._, 1805), and Etienne

Geoffroy Saint-Hilaire (_Gilb. Ann._, 1803) pursued the subject with

success; and Henry Cavendish (_Phil. Trans._, 1776) constructed an

artificial torpedo, by which he imitated the actions of the living

animal. The subject was also investigated (_Phil. Trans._, 1812, 1817)

by Dr T.J. Todd (1789-1840), Sir Humphry Davy (id. 1829), John Davy (id.

1832, 1834, 1841) and Faraday (_Exp. Res._, vol. ii.). The power of

giving electric shocks has been discovered also in the _Gymnotus

electricus_ (electric eel), the _Malapterurus electricus_, the

_Trichiurus electricus_, and the _Tetraodon electricus_. The most

interesting and the best known of these singular fishes is the

_Gymnotus_ or Surinam eel. Humboldt gives a very graphic account of the

combats which are carried on in South America between the gymnoti and

the wild horses in the vicinity of Calabozo.

_Cavendish's Researches._--The work of Henry Cavendish (1731-1810)

entitles him to a high place in the list of electrical investigators. A

considerable part of Cavendish's work was rescued from oblivion in 1879

and placed in an easily accessible form by Professor Clerk Maxwell, who

edited the original manuscripts in the possession of the duke of

Devonshire.[4] Amongst Cavendish's important contributions were his

exact measurements of electrical capacity. The leading idea which

distinguishes his work from that of his predecessors was his use of the

phrase "degree of electrification" with a clear scientific definition

which shows it to be equivalent in meaning to the modern term "electric

potential." Cavendish compared the capacity of different bodies with

those of conducting spheres of known diameter and states these

capacities in "globular inches," a globular inch being the capacity of a

sphere 1 in. in diameter. Hence his measurements are all directly

comparable with modern electrostatic measurements in which the unit of

capacity is that of a sphere 1 centimetre in radius. Cavendish measured

the capacity of disks and condensers of various forms, and proved that

the capacity of a Leyden pane is proportional to the surface of the

tinfoil and inversely as the thickness of the glass. In connexion with

this subject he anticipated one of Faraday's greatest discoveries,

namely, the effect of the dielectric or insulator upon the capacity of a

condenser formed with it, in other words, made the discovery of specific

inductive capacity (see _Electrical Researches_, p. 183). He made many

measurements of the electric conductivity of different solids and

liquids, by comparing the intensity of the electric shock taken through

his body and various conductors. He seems in this way to have educated

in himself a very precise "electrical sense," making use of his own

nervous system as a kind of physiological galvanometer. One of the most

important investigations he made in this way was to find out, as he

expressed it, "what power of the velocity the resistance is proportional

to." Cavendish meant by the term "velocity" what we now call the

current, and by "resistance" the electromotive force which maintains the

current. By various experiments with liquids in tubes he found this

power was nearly unity. This result thus obtained by Cavendish in

January 1781, that the current varies in direct proportion to the

electromotive force, was really an anticipation of the fundamental law

of electric flow, discovered independently by G.S. Ohm in 1827, and

since known as Ohm's Law. Cavendish also enunciated in 1776 all the laws

of division of electric current between circuits in parallel, although

they are generally supposed to have been first given by Sir C.

Wheatstone. Another of his great investigations was the determination of

the law according to which electric force varies with the distance.

Starting from the fact that if an electrified globe, placed within two

hemispheres which fit over it without touching, is brought in contact

with these hemispheres, it gives up the whole of its charge to them--in

other words, that the charge on an electrified body is wholly on the

surface--he was able to deduce by most ingenious reasoning the law that

electric force varies inversely as the square of the distance. The

accuracy of his measurement, by which he established within 2% the above

law, was only limited by the sensibility, or rather insensibility, of

the pith ball electrometer, which was his only means of detecting the

electric charge.[5] In the accuracy of his quantitative measurements and

the range of his researches and his combination of mathematical and

physical knowledge, Cavendish may not inaptly be described as the Kelvin

of the 18th century. Nothing but his curious indifference to the

publication of his work prevented him from securing earlier recognition

for it.

_Coulomb's Work._--Contemporary with Cavendish was C.A. Coulomb

(1736-1806), who in France addressed himself to the same kind of exact

quantitative work as Cavendish in England. Coulomb has made his name for

ever famous by his invention and application of his torsion balance to

the experimental verification of the fundamental law of electric

attraction, in which, however, he was anticipated by Cavendish, namely,

that the force of attraction between two small electrified spherical

bodies varies as the product of their charges and inversely as the

square of the distance of their centres. Coulomb's work received better

publication than Cavendish's at the time of its accomplishment, and

provided a basis on which mathematicians could operate. Accordingly the

close of the 18th century drew into the arena of electrical

investigation on its mathematical side P.S. Laplace, J.B. Biot, and

above all, S.D. Poisson. Adopting the hypothesis of two fluids, Coulomb

investigated experimentally and theoretically the distribution of

electricity on the surface of bodies by means of his proof plane. He

determined the law of distribution between two conducting bodies in

contact; and measured with his proof plane the density of the

electricity at different points of two spheres in contact, and

enunciated an important law. He ascertained the distribution of

electricity among several spheres (whether equal or unequal) placed in

contact in a straight line; and he measured the distribution of

electricity on the surface of a cylinder, and its distribution between

a sphere and cylinder of different lengths but of the same diameter. His

experiments on the dissipation of electricity possess also a high value.

He found that the momentary dissipation was proportional to the degree

of electrification at the time, and that, when the charge was moderate,

its dissipation was not altered in bodies of different kinds or shapes.

The temperature and pressure of the atmosphere did not produce any

sensible change; but he concluded that the dissipation was nearly

proportional to the cube of the quantity of moisture in the air.[6] In

examining the dissipation which takes place along imperfectly insulating

substances, he found that a thread of gum-lac was the most perfect of

all insulators; that it insulated ten times as well as a dry silk

thread; and that a silk thread covered with fine sealing-wax insulated

as powerfully as gum-lac when it had four times its length. He found

also that the dissipation of electricity along insulators was chiefly

owing to adhering moisture, but in some measure also to a slight

conducting power. For his memoirs see _Mem. de math. et phys. de l'acad.

de sc._, 1785, &c.

SECOND PERIOD.--We now enter upon the second period of electrical

research inaugurated by the epoch-making discovery of Alessandro Volta

(1745-1827). L. Galvani had made in 1790 his historic observations on

the muscular contraction produced in the bodies of recently killed frogs

when an electrical machine was being worked in the same room, and

described them in 1791 (_De viribus electricitatis in motu musculari

commentarius_, Bologna, 1791). Volta followed up these observations with

rare philosophic insight and experimental skill. He showed that all

conductors liquid and solid might be divided into two classes which he

called respectively conductors of the first and of the second class, the

first embracing metals and carbon in its conducting form, and the second

class, water, aqueous solutions of various kinds, and generally those

now called electrolytes. In the case of conductors of the first class he

proved by the use of the condensing electroscope, aided probably by some

form of multiplier or doubler, that a difference of potential (see

ELECTROSTATICS) was created by the mere contact of two such conductors,

one of them being positively electrified and the other negatively. Volta

showed, however, that if a series of bodies of the first class, such as

disks of various metals, are placed in contact, the potential difference

between the first and the last is just the same as if they are

immediately in contact. There is no accumulation of potential. If,

however, pairs of metallic disks, made, say, of zinc and copper, are

alternated with disks of cloth wetted with a conductor of the second

class, such, for instance, as dilute acid or any electrolyte, then the

effect of the feeble potential difference between one pair of copper and

zinc disks is added to that of the potential difference between the next

pair, and thus by a sufficiently long series of pairs any required

difference of potential can be accumulated.

_The Voltaic Pile._--This led him about 1799 to devise his famous

voltaic pile consisting of disks of copper and zinc or other metals with

wet cloth placed between the pairs. Numerous examples of Volta's

original piles at one time existed in Italy, and were collected together

for an exhibition held at Como in 1899, but were unfortunately destroyed

by a disastrous fire on the 8th of July 1899. Volta's description of his

pile was communicated in a letter to Sir Joseph Banks, president of the

Royal Society of London, on the 20th of March 1800, and was printed in

the _Phil. Trans._, vol. 90, pt. 1, p. 405. It was then found that when

the end plates of Volta's pile were connected to an electroscope the

leaves diverged either with positive or negative electricity. Volta also

gave his pile another form, the _couronne des tasses_ (crown of cups),

in which connected strips of copper and zinc were used to bridge between

cups of water or dilute acid. Volta then proved that all metals could be

arranged in an electromotive series such that each became positive when

placed in contact with the one next below it in the series. The origin

of the electromotive force in the pile has been much discussed, and

Volta's discoveries gave rise to one of the historic controversies of

science. Volta maintained that the mere contact of metals was sufficient

to produce the electrical difference of the end plates of the pile. The

discovery that chemical action was involved in the process led to the

advancement of the chemical theory of the pile and this was strengthened

by the growing insight into the principle of the conservation of energy.

In 1851 Lord Kelvin (Sir W. Thomson), by the use of his then

newly-invented electrometer, was able to confirm Volta's observations on

contact electricity by irrefutable evidence, but the contact theory of

the voltaic pile was then placed on a basis consistent with the

principle of the conservation of energy. A.A. de la Rive and Faraday

were ardent supporters of the chemical theory of the pile, and even at

the present time opinions of physicists can hardly be said to be in

entire accordance as to the source of the electromotive force in a

voltaic couple or pile.[7]

Improvements in the form of the voltaic pile were almost immediately

made by W. Cruickshank (1745-1800), Dr W.H. Wollaston and Sir H. Davy,

and these, together with other eminent continental chemists, such as

A.F. de Fourcroy, L.J. Thenard and J.W. Ritter (1776-1810), ardently

prosecuted research with the new instrument. One of the first

discoveries made with it was its power to electrolyse or chemically

decompose certain solutions. William Nicholson (1753-1815) and Sir

Anthony Carlisle (1768-1840) in 1800 constructed a pile of silver and

zinc plates, and placing the terminal wires in water noticed the

evolution from these wires of bubbles of gas, which they proved to be

oxygen and hydrogen. These two gases, as Cavendish and James Watt had

shown in 1784, were actually the constituents of water. From that date

it was clearly recognized that a fresh implement of great power had been

given to the chemist. Large voltaic piles were then constructed by

Andrew Crosse (1784-1855) and Sir H. Davy, and improvements initiated by

Wollaston and Robert Hare (1781-1858) of Philadelphia. In 1806 Davy

communicated to the Royal Society of London a celebrated paper on some

"Chemical Agencies of Electricity," and after providing himself at the

Royal Institution of London with a battery of several hundred cells, he

announced in 1807 his great discovery of the electrolytic decomposition

of the alkalis, potash and soda, obtaining therefrom the metals

potassium and sodium. In July 1808 Davy laid a request before the

managers of the Royal Institution that they would set on foot a

subscription for the purchase of a specially large voltaic battery; as a

result he was provided with one of 2000 pairs of plates, and the first

experiment performed with it was the production of the electric arc

light between carbon poles. Davy followed up his initial work with a

long and brilliant series of electrochemical investigations described

for the most part in the _Phil. Trans._ of the Royal Society.

_Magnetic Action of Electric Current._--Noticing an analogy between the

polarity of the voltaic pile and that of the magnet, philosophers had

long been anxious to discover a relation between the two, but twenty

years elapsed after the invention of the pile before Hans Christian

Oersted (1777-1851), professor of natural philosophy in the university

of Copenhagen, made in 1819 the discovery which has immortalized his

name. In the _Annals of Philosophy_ (1820, 16, p. 273) is to be found an

English translation of Oersted's original Latin essay (entitled

"Experiments on the Effect of a Current of Electricity on the Magnetic

Needle"), dated the 21st of July 1820, describing his discovery. In it

Oersted describes the action he considers is taking place around the

conductor joining the extremities of the pile; he speaks of it as the

electric conflict, and says: "It is sufficiently evident that the

electric conflict is not confined to the conductor, but is dispersed

pretty widely in the circumjacent space. We may likewise conclude that

this conflict performs circles round the wire, for without this

condition it seems impossible that one part of the wire when placed

below the magnetic needle should drive its pole to the east, and when

placed above it, to the west." Oersted's important discovery was the

fact that when a wire joining the end plates of a voltaic pile is held

near a pivoted magnet or compass needle, the latter is deflected and

places itself more or less transversely to the wire, the direction

depending upon whether the wire is above or below the needle, and on the

manner in which the copper or zinc ends of the pile are connected to it.

It is clear, moreover, that Oersted clearly recognized the existence of

what is now called the magnetic field round the conductor. This

discovery of Oersted, like that of Volta, stimulated philosophical

investigation in a high degree.

_Electrodynamics._--On the 2nd of October 1820, A.M. Ampere presented to

the French Academy of Sciences an important memoir,[8] in which he

summed up the results of his own and D.F.J. Arago's previous

investigations in the new science of electromagnetism, and crowned that

labour by the announcement of his great discovery of the dynamical

action between conductors conveying the electric currents. Ampere in

this paper gave an account of his discovery that conductors conveying

electric currents exercise a mutual attraction or repulsion on one

another, currents flowing in the same direction in parallel conductors

attracting, and those in opposite directions repelling. Respecting this

achievement when developed in its experimental and mathematical

completeness, Clerk Maxwell says that it was "perfect in form and

unassailable in accuracy." By a series of well-chosen experiments Ampere

established the laws of this mutual action, and not only explained

observed facts by a brilliant train of mathematical analysis, but

predicted others subsequently experimentally realized. These

investigations led him to the announcement of the fundamental law of

action between elements of current, or currents in infinitely short

lengths of linear conductors, upon one another at a distance; summed up

in compact expression this law states that the action is proportional to

the product of the current strengths of the two elements, and the

lengths of the two elements, and inversely proportional to the square of

the distance between the two elements, and also directly proportional to

a function of the angles which the line joining the elements makes with

the directions of the two elements respectively. Nothing is more

remarkable in the history of discovery than the manner in which Ampere

seized upon the right clue which enabled him to disentangle the

complicated phenomena of electrodynamics and to deduce them all as a

consequence of one simple fundamental law, which occupies in

electrodynamics the position of the Newtonian law of gravitation in

physical astronomy.

In 1821 Michael Faraday (1791-1867), who was destined later on to do so

much for the science of electricity, discovered electromagnetic

rotation, having succeeded in causing a wire conveying a voltaic current

to rotate continuously round the pole of a permanent magnet.[9] This

experiment was repeated in a variety of forms by A.A. De la Rive, Peter

Barlow (1776-1862), William Ritchie (1790-1837), William Sturgeon

(1783-1850), and others; and Davy (_Phil. Trans._, 1823) showed that

when two wires connected with the pole of a battery were dipped into a

cup of mercury placed on the pole of a powerful magnet, the fluid

rotated in opposite directions about the two electrodes.

_Electromagnetism._--In 1820 Arago (_Ann. Chim. Phys._, 1820, 15, p. 94)

and Davy (_Annals of Philosophy_, 1821) discovered independently the

power of the electric current to magnetize iron and steel. Felix Savary

(1797-1841) made some very curious observations in 1827 on the

magnetization of steel needles placed at different distances from a wire

conveying the discharge of a Leyden jar (_Ann. Chim. Phys._, 1827, 34).

W. Sturgeon in 1824 wound a copper wire round a bar of iron bent in the

shape of a horseshoe, and passing a voltaic current through the wire

showed that the iron became powerfully magnetized as long as the

connexion with the pile was maintained (_Trans. Soc. Arts_, 1825). These

researches gave us the electromagnet, almost as potent an instrument of

research and invention as the pile itself (see ELECTROMAGNETISM).

Ampere had already previously shown that a spiral conductor or solenoid

when traversed by an electric current possesses magnetic polarity, and

that two such solenoids act upon one another when traversed by electric

currents as if they were magnets. Joseph Henry, in the United States,

first suggested the construction of what were then called intensity

electromagnets, by winding upon a horseshoe-shaped piece of soft iron

many superimposed windings of copper wire, insulated by covering it with

silk or cotton, and then sending through the coils the current from a

voltaic battery. The dependence of the intensity of magnetization on the

strength of the current was subsequently investigated (_Pogg. Ann.

Phys._, 1839, 47) by H.F.E. Lenz (1804-1865) and M.H. von Jacobi

(1801-1874). J.P. Joule found that magnetization did not increase

proportionately with the current, but reached a maximum (_Sturgeon's

Annals of Electricity_, 1839, 4). Further investigations on this subject

were carried on subsequently by W.E. Weber (1804-1891), J.H.J. Muller

(1809-1875), C.J. Dub (1817-1873), G.H. Wiedemann (1826-1899), and

others, and in modern times by H.A. Rowland (1848-1901), Shelford

Bidwell (b. 1848), John Hopkinson (1849-1898), J.A. Ewing (b. 1855) and

many others. Electric magnets of great power were soon constructed in

this manner by Sturgeon, Joule, Henry, Faraday and Brewster. Oersted's

discovery in 1819 was indeed epoch-making in the degree to which it

stimulated other research. It led at once to the construction of the

galvanometer as a means of detecting and measuring the electric current

in a conductor. In 1820 J.S.C. Schweigger (1779-1857) with his

"multiplier" made an advance upon Oersted's discovery, by winding the

wire conveying the electric current many times round the pivoted

magnetic needle and thus increasing the deflection; and L. Nobili

(1784-1835) in 1825 conceived the ingenious idea of neutralizing the

directive effect of the earth's magnetism by employing a pair of

magnetized steel needles fixed to one axis, but with their magnetic

poles pointing in opposite directions. Hence followed the astatic

multiplying galvanometer.

_Electrodynamic Rotation._--The study of the relation between the magnet

and the circuit conveying an electric current then led Arago to the

discovery of the "magnetism of rotation." He found that a vibrating

magnetic compass needle came to rest sooner when placed over a plate of

copper than otherwise, and also that a plate of copper rotating under a

suspended magnet tended to drag the magnet in the same direction. The

matter was investigated by Charles Babbage, Sir J.F.W. Herschel, Peter

Barlow and others, but did not receive a final explanation until after

the discovery of electromagnetic induction by Faraday in 1831. Ampere's

investigations had led electricians to see that the force acting upon a

magnetic pole due to a current in a neighbouring conductor was such as

to tend to cause the pole to travel round the conductor. Much ingenuity

had, however, to be expended before a method was found of exhibiting

such a rotation. Faraday first succeeded by the simple but ingenious

device of using a light magnetic needle tethered flexibly to the bottom

of a cup containing mercury so that one pole of the magnet was just

above the surface of the mercury. On bringing down on to the mercury

surface a wire conveying an electric current, and allowing the current

to pass through the mercury and out at the bottom, the magnetic pole at

once began to rotate round the wire (_Exper. Res._, 1822, 2, p. 148).

Faraday and others then discovered, as already mentioned, means to make

the conductor conveying the current rotate round a magnetic pole, and

Ampere showed that a magnet could be made to rotate on its own axis when

a current was passed through it. The difficulty in this case consisted

in discovering means by which the current could be passed through one

half of the magnet without passing it through the other half. This,

however, was overcome by sending the current out at the centre of the

magnet by means of a short length of wire dipping into an annular groove

containing mercury. Barlow, Sturgeon and others then showed that a

copper disk could be made to rotate between the poles of a horseshoe

magnet when a current was passed through the disk from the centre to the

circumference, the disk being rendered at the same time freely movable

by making a contact with the circumference by means of a mercury trough.

These experiments furnished the first elementary forms of electric

motor, since it was then seen that rotatory motion could be produced in

masses of metal by the mutual action of conductors conveying electric

current and magnetic fields. By his discovery of thermo-electricity in

1822 (_Pogg. Ann. Phys._, 6), T.J. Seebeck (1770-1831) opened up a new

region of research (see THERMOELECTRICITY). James Cumming (1777-1861) in

1823 (_Annals of Philosophy_, 1823) found that the thermo-electric

series varied with the temperature, and J.C.A. Peltier (1785-1845) in

1834 discovered that a current passed across the junction of two metals

either generated or absorbed heat.

_Ohm's Law._--In 1827 Dr G.S. Ohm (1787-1854) rendered a great service

to electrical science by his mathematical investigation of the voltaic

circuit, and publication of his paper, _Die galvanische Kette

mathematisch bearbeitet_. Before his time, ideas on the measurable

quantities with which we are concerned in an electric circuit were

extremely vague. Ohm introduced the clear idea of current strength as an

effect produced by electromotive force acting as a cause in a circuit

having resistance as its quality, and showed that the current was

directly proportional to the electromotive force and inversely as the

resistance. Ohm's law, as it is called, was based upon an analogy with

the flow of heat in a circuit, discussed by Fourier. Ohm introduced the

definite conception of the distribution along the circuit of

"electroscopic force" or tension (_Spannung_), corresponding to the

modern term potential. Ohm verified his law by the aid of

thermo-electric piles as sources of electromotive force, and Davy,

C.S.M. Pouillet (1791-1868), A.C. Becquerel (1788-1878), G.T. Fechner

(1801-1887), R.H.A. Kohlrausch (1809-1858) and others laboured at its

confirmation. In more recent times, 1876, it was rigorously tested by G.

Chrystal (b. 1851) at Clerk Maxwell's instigation (see _Brit. Assoc.

Report_, 1876, p. 36), and although at its original enunciation its

meaning was not at first fully apprehended, it soon took its place as

the expression of the fundamental law of electrokinetics.

_Induction of Electric Currents._--In 1831 Faraday began the

investigations on electromagnetic induction which proved more fertile in

far-reaching practical consequences than any of those which even his

genius gave to the world. These advances all centre round his supreme

discovery of the induction of electric currents. Fully familiar with the

fact that an electric charge upon one conductor could produce a charge

of opposite sign upon a neighbouring conductor, Faraday asked himself

whether an electric current passing through a conductor could not in any

like manner induce an electric current in some neighbouring conductor.

His first experiments on this subject were made in the month of November

1825, but it was not until the 29th of August 1831 that he attained

success. On that date he had provided himself with an iron ring, over

which he had wound two coils of insulated copper wire. One of these

coils was connected with the voltaic battery and the other with the

galvanometer. He found that at the moment the current in the battery

circuit was started or stopped, transitory currents appeared in the

galvanometer circuit in opposite directions. In ten days of brilliant

investigation, guided by clear insight from the very first into the

meaning of the phenomena concerned, he established experimentally the

fact that a current may be induced in a conducting circuit simply by the

variation in a magnetic field, the lines of force of which are linked

with that circuit. The whole of Faraday's investigations on this

subject can be summed up in the single statement that if a conducting

circuit is placed in a magnetic field, and if either by variation of the

field or by movement or variation of the form of the circuit the total

magnetic flux linked with the circuit is varied, an electromotive force

is set up in that circuit which at any instant is measured by the rate

at which the total flux linked with the circuit is changing.

Amongst the memorable achievements of the ten days which Faraday devoted

to this investigation was the discovery that a current could be induced

in a conducting wire simply by moving it in the neighbourhood of a

magnet. One form which this experiment took was that of rotating a

copper disk between the poles of a powerful electric magnet. He then

found that a conductor, the ends of which were connected respectively

with the centre and edge of the disk, was traversed by an electric

current. This important fact laid the foundation for all subsequent

inventions which finally led to the production of electromagnetic or

dynamo-electric machines.

THIRD PERIOD.--With this supremely important discovery of Faraday's we

enter upon the third period of electrical research, in which that

philosopher himself was the leading figure. He not only collected the

facts concerning electromagnetic induction so industriously that nothing

of importance remained for future discovery, and embraced them all in

one law of exquisite simplicity, but he introduced his famous conception

of lines of force which changed entirely the mode of regarding

electrical phenomena. The French mathematicians, Coulomb, Biot, Poisson

and Ampere, had been content to accept the fact that electric charges or

currents in conductors could exert forces on other charges or conductors

at a distance without inquiring into the means by which this action at a

distance was produced. Faraday's mind, however, revolted against this

notion; he felt intuitively that these distance actions must be the

result of unseen operations in the interposed medium. Accordingly when

he sprinkled iron filings on a card held over a magnet and revealed the

curvilinear system of lines of force (see MAGNETISM), he regarded these

fragments of iron as simple indicators of a physical state in the space

already in existence round the magnet. To him a magnet was not simply a

bar of steel; it was the core and origin of a system of lines of

magnetic force attached to it and moving with it. Similarly he came to

see an electrified body as a centre of a system of lines of

electrostatic force. All the space round magnets, currents and electric

charges was therefore to Faraday the seat of corresponding lines of

magnetic or electric force. He proved by systematic experiments that the

electromotive forces set up in conductors by their motions in magnetic

fields or by the induction of other currents in the field were due to

the secondary conductor _cutting_ lines of magnetic force. He invented

the term "electrotonic state" to signify the total magnetic flux due to

a conductor conveying a current, which was linked with any secondary

circuit in the field or even with itself.

_Faraday's Researches._--Space compels us to limit our account of the

scientific work done by Faraday in the succeeding twenty years, in

elucidating electrical phenomena and adding to the knowledge thereon, to

the very briefest mention. We must refer the reader for further

information to his monumental work entitled _Experimental Researches on

Electricity_, in three volumes, reprinted from the _Phil. Trans._

between 1831 and 1851. Faraday divided these researches into various

series. The 1st and 2nd concern the discovery of magneto-electric

induction already mentioned. The 3rd series (1833) he devoted to

discussion of the identity of electricity derived from various sources,

frictional, voltaic, animal and thermal, and he proved by rigorous

experiments the identity and similarity in properties of the electricity

generated by these various methods. The 5th series (1833) is occupied

with his electrochemical researches. In the 7th series (1834) he defines

a number of new terms, such as electrolyte, electrolysis, anode and

cathode, &c., in connexion with electrolytic phenomena, which were

immediately adopted into the vocabulary of science. His most important

contribution at this date was the invention of the voltameter and his

enunciation of the laws of electrolysis. The voltameter provided a means

of measuring quantity of electricity, and in the hands of Faraday and

his successors became an appliance of fundamental importance. The 8th

series is occupied with a discussion of the theory of the voltaic pile,

in which Faraday accumulates evidence to prove that the source of the

energy of the pile must be chemical. He returns also to this subject in

the 16th series. In the 9th series (1834) he announced the discovery of

the important property of electric conductors, since called their

self-induction or inductance, a discovery in which, however, he was

anticipated by Joseph Henry in the United States. The 11th series (1837)

deals with electrostatic induction and the statement of the important

fact of the specific inductive capacity of insulators or dielectrics.

This discovery was made in November 1837 when Faraday had no knowledge

of Cavendish's previous researches into this matter. The 19th series

(1845) contains an account of his brilliant discovery of the rotation of

the plane of polarized light by transparent dielectrics placed in a

magnetic field, a relation which established for the first time a

practical connexion between the phenomena of electricity and light. The

20th series (1845) contains an account of his researches on the

universal action of magnetism and diamagnetic bodies. The 22nd series

(1848) is occupied with the discussion of magneto-crystallic force and

the abnormal behaviour of various crystals in a magnetic field. In the

25th series (1850) he made known his discovery of the magnetic character

of oxygen gas, and the important principle that the terms paramagnetic

and diamagnetic are relative. In the 26th series (1850) he returned to a

discussion of magnetic lines of force, and illuminated the whole subject

of the magnetic circuit by his transcendent insight into the intricate

phenomena concerned. In 1855 he brought these researches to a conclusion

by a general article on magnetic philosophy, having placed the whole

subject of magnetism and electromagnetism on an entirely novel and solid

basis. In addition to this he provided the means for studying the

phenomena not only qualitatively, but also quantitatively, by the

profoundly ingenious instruments he invented for that purpose.

_Electrical Measurement._--Faraday's ideas thus pressed upon

electricians the necessity for the quantitative measurement of

electrical phenomena.[10] It has been already mentioned that Schweigger

invented in 1820 the "multiplier," and Nobili in 1825 the astatic

galvanometer. C.S.M. Pouillet in 1837 contributed the sine and tangent

compass, and W.E. Weber effected great improvements in them and in the

construction and use of galvanometers. In 1849 H. von Helmholtz devised

a tangent galvanometer with two coils. The measurement of electric

resistance then engaged the attention of electricians. By his Memoirs in

the _Phil. Trans._ in 1843, Sir Charles Wheatstone gave a great impulse

to this study. He invented the rheostat and improved the resistance

balance, invented by S.H. Christie (1784-1865) in 1833, and subsequently

called the Wheatstone Bridge. (See his _Scientific Papers_, published by

the Physical Society of London, p. 129.) Weber about this date invented

the electrodynamometer, and applied the mirror and scale method of

reading deflections, and in co-operation with C.F. Gauss introduced a

system of absolute measurement of electric and magnetic phenomena. In

1846 Weber proceeded with improved apparatus to test Ampere's laws of

electrodynamics. In 1845 H.G. Grassmann (1809-1877) published (_Pogg.

Ann._ vol. 64) his "Neue Theorie der Electrodynamik," in which he gave

an elementary law differing from that of Ampere but leading to the same

results for closed circuits. In the same year F.E. Neumann published

another law. In 1846 Weber announced his famous hypothesis concerning

the connexion of electrostatic and electrodynamic phenomena. The work of

Neumann and Weber had been stimulated by that of H.F.E. Lenz

(1804-1865), whose researches (_Pogg. Ann._, 1834, 31; 1835, 34) among

other results led him to the statement of the law by means of which the

direction of the induced current can be predicted from the theory of

Ampere, the rule being that the direction of the induced current is

always such that its electrodynamic action tends to oppose the motion

which produces it.

Neumann in 1845 did for electromagnetic induction what Ampere did for

electrodynamics, basing his researches upon the experimental laws of

Lenz. He discovered a function, which has been called the potential of

one circuit on another, from which he deduced a theory of induction

completely in accordance with experiment. Weber at the same time deduced

the mathematical laws of induction from his elementary law of electrical

action, and with his improved instruments arrived at accurate

verifications of the law of induction, which by this time had been

developed mathematically by Neumann and himself. In 1849 G.R. Kirchhoff

determined experimentally in a certain case the absolute value of the

current induced by one circuit in another, and in the same year Erik

Edland (1819-1888) made a series of careful experiments on the induction

of electric currents which further established received theories. These

labours laid the foundation on which was subsequently erected a complete

system for the absolute measurement of electric and magnetic quantities,

referring them all to the fundamental units of mass, length and time.

Helmholtz gave at the same time a mathematical theory of induced

currents and a valuable series of experiments in support of them (_Pogg.

Ann._, 1851). This great investigator and luminous expositor just before

that time had published his celebrated essay, _Die Erhaltung der Kraft_

("The Conservation of Energy"), which brought to a focus ideas which had

been accumulating in consequence of the work of J.P. Joule, J.R. von

Mayer and others, on the transformation of various forms of physical

energy, and in particular the mechanical equivalent of heat. Helmholtz

brought to bear upon the subject not only the most profound mathematical

attainments, but immense experimental skill, and his work in connexion

with this subject is classical.

_Lord Kelvin's Work._--About 1842 Lord Kelvin (then William Thomson)

began that long career of theoretical and practical discovery and

invention in electrical science which revolutionized every department of

pure and applied electricity. His early contributions to electrostatics

and electrometry are to be found described in his _Reprint of Papers on

Electrostatics and Magnetism_ (1872), and his later work in his

collected _Mathematical and Physical Papers_. By his studies in

electrostatics, his elegant method of electrical images, his development

of the theory of potential and application of the principle of

conservation of energy, as well as by his inventions in connexion with

electrometry, he laid the foundations of our modern knowledge of

electrostatics. His work on the electrodynamic qualities of metals,

thermo-electricity, and his contributions to galvanometry, were not less

massive and profound. From 1842 onwards to the end of the 19th century,

he was one of the great master workers in the field of electrical

discovery and research.[11] In 1853 he published a paper "On Transient

Electric Currents" (_Phil. Mag._, 1853 [4], 5, p. 393), in which he

applied the principle of the conservation of energy to the discharge of

a Leyden jar. He added definiteness to the idea of the self-induction or

inductance of an electric circuit, and gave a mathematical expression

for the current flowing out of a Leyden jar during its discharge. He

confirmed an opinion already previously expressed by Helmholtz and by

Henry, that in some circumstances this discharge is oscillatory in

nature, consisting of an alternating electric current of high frequency.

These theoretical predictions were confirmed and others, subsequently,

by the work of B.W. Feddersen (b. 1832), C.A. Paalzow (b. 1823), and it

was then seen that the familiar phenomena of the discharge of a Leyden

jar provided the means of generating electric oscillations of very high

frequency.

_Telegraphy._--Turning to practical applications of electricity, we may

note that electric telegraphy took its rise in 1820, beginning with a

suggestion of Ampere immediately after Oersted's discovery. It was

established by the work of Weber and Gauss at Gottingen in 1836, and

that of C.A. Steinheil (1801-1870) of Munich, Sir W.F. Cooke (1806-1879)

and Sir C. Wheatstone in England, Joseph Henry and S.F.B. Morse

(1791-1872) in the United States in 1837. In 1845 submarine telegraphy

was inaugurated by the laying of an insulated conductor across the

English Channel by the brothers Brett, and their temporary success was

followed by the laying in 1851 of a permanent Dover-Calais cable by T.R.

Crampton. In 1856 the project for an Atlantic submarine cable took shape

and the Atlantic Telegraph Company was formed with a capital of

L350,000, with Sir Charles Bright as engineer-in-chief and E.O.W.

Whitehouse as electrician. The phenomena connected with the propagation

of electric signals by underground insulated wires had already engaged

the attention of Faraday in 1854, who pointed out the Leyden-jar-like

action of an insulated subterranean wire. Scientific and practical

questions connected with the possibility of laying an Atlantic submarine

cable then began to be discussed, and Lord Kelvin was foremost in

developing true scientific knowledge on this subject, and in the

invention of appliances for utilizing it. One of his earliest and most

useful contributions (in 1858) was the invention of the mirror

galvanometer. Abandoning the long and somewhat heavy magnetic needles

that had been used up to that date in galvanometers, he attached to the

back of a very small mirror made of microscopic glass a fragment of

magnetized watch-spring, and suspended the mirror and needle by means of

a cocoon fibre in the centre of a coil of insulated wire. By this simple

device he provided a means of measuring small electric currents far in

advance of anything yet accomplished, and this instrument proved not

only most useful in pure scientific researches, but at the same time was

of the utmost value in connexion with submarine telegraphy. The history

of the initial failures and final success in laying the Atlantic cable

has been well told by Mr. Charles Bright (see _The Story of the Atlantic

Cable_, London, 1903).[12] The first cable laid in 1857 broke on the

11th of August during laying. The second attempt in 1858 was successful,

but the cable completed on the 5th of August 1858 broke down on the 20th

of October 1858, after 732 messages had passed through it. The third

cable laid in 1865 was lost on the 2nd of August 1865, but in 1866 a

final success was attained and the 1865 cable also recovered and

completed. Lord Kelvin's mirror galvanometer was first used in receiving

signals through the short-lived 1858 cable. In 1867 he invented his

beautiful siphon-recorder for receiving and recording the signals

through long cables. Later, in conjunction with Prof. Fleeming Jenkin,

he devised his automatic curb sender, an appliance for sending signals

by means of punched telegraphic paper tape. Lord Kelvin's contributions

to the science of exact electric measurement[13] were enormous. His

ampere-balances, voltmeters and electrometers, and double bridge, are

elsewhere described in detail (see AMPEREMETER; ELECTROMETER, and

WHEATSTONE'S BRIDGE).

_Dynamo._--The work of Faraday from 1831 to 1851 stimulated and

originated an immense mass of scientific research, but at the same time

practical inventors had not been slow to perceive that it was capable of

purely technical application. Faraday's copper disk rotated between the

poles of a magnet, and producing thereby an electric current, became the

parent of innumerable machines in which mechanical energy was directly

converted into the energy of electric currents. Of these machines,

originally called magneto-electric machines, one of the first was

devised in 1832 by H. Pixii. It consisted of a fixed horseshoe armature

wound over with insulated copper wire in front of which revolved about a

vertical axis a horseshoe magnet. Pixii, who invented the split tube

commutator for converting the alternating current so produced into a

continuous current in the external circuit, was followed by J. Saxton,

E.M. Clarke, and many others in the development of the above-described

magneto-electric machine. In 1857 E.W. Siemens effected a great

improvement by inventing a shuttle armature and improving the shape of

the field magnet. Subsequently similar machines with electromagnets were

introduced by Henry Wilde (b. 1833), Siemens, Wheatstone, W. Ladd and

others, and the principle of self-excitation was suggested by Wilde,

C.F. Varley (1828-1883), Siemens and Wheatstone (see DYNAMO). These

machines about 1866 and 1867 began to be constructed on a commercial

scale and were employed in the production of the electric light. The

discovery of electric-current induction also led to the production of

the induction coil (q.v.), improved and brought to its present

perfection by W. Sturgeon, E.R. Ritchie, N.J. Callan, H.D. Ruhmkorff

(1803-1877), A.H.L. Fizeau, and more recently by A. Apps and modern

inventors. About the same time Fizeau and J.B.L. Foucault devoted

attention to the invention of automatic apparatus for the production of

Davy's electric arc (see LIGHTING: _ELECTRIC_), and these appliances in

conjunction with magneto-electric machines were soon employed in

lighthouse work. With the advent of large magneto-electric machines the

era of electrotechnics was fairly entered, and this period, which may be

said to terminate about 1867 to 1869, was consummated by the theoretical

work of Clerk Maxwell.

_Maxwell's Researches._--James Clerk Maxwell (1831-1879) entered on his

electrical studies with a desire to ascertain if the ideas of Faraday,

so different from those of Poisson and the French mathematicians, could

be made the foundation of a mathematical method and brought under the

power of analysis.[14] Maxwell started with the conception that all

electric and magnetic phenomena are due to effects taking place in the

dielectric or in the ether if the space be vacuous. The phenomena of

light had compelled physicists to postulate a space-filling medium, to

which the name ether had been given, and Henry and Faraday had long

previously suggested the idea of an electromagnetic medium. The

vibrations of this medium constitute the agency called light. Maxwell

saw that it was unphilosophical to assume a multiplicity of ethers or

media until it had been proved that one would not fulfil all the

requirements. He formulated the conception, therefore, of electric

charge as consisting in a displacement taking place in the dielectric or

electromagnetic medium (see ELECTROSTATICS). Maxwell never committed

himself to a precise definition of the physical nature of electric

displacement, but considered it as defining that which Faraday had

called the polarization in the insulator, or, what is equivalent, the

number of lines of electrostatic force passing normally through a unit

of area in the dielectric. A second fundamental conception of Maxwell

was that the electric displacement whilst it is changing is in effect an

electric current, and creates, therefore, magnetic force. The total

current at any point in a dielectric must be considered as made up of

two parts: first, the true conduction current, if it exists; and second,

the rate of change of dielectric displacement. The fundamental fact

connecting electric currents and magnetic fields is that the line

integral of magnetic force taken once round a conductor conveying an

electric current is equal to 4 [pi]-times the surface integral of the

current density, or to 4 [pi]-times the total current flowing through

the closed line round which the integral is taken (see ELECTROKINETICS).

A second relation connecting magnetic and electric force is based upon

Faraday's fundamental law of induction, that the rate of change of the

total magnetic flux linked with a conductor is a measure of the

electromotive force created in it (see ELECTROKINETICS). Maxwell also

introduced in this connexion the notion of the vector potential.

Coupling together these ideas he was finally enabled to prove that the

propagation of electric and magnetic force takes place through space

with a certain velocity determined by the dielectric constant and the

magnetic permeability of the medium. To take a simple instance, if we

consider an electric current as flowing in a conductor it is, as Oersted

discovered, surrounded by closed lines of magnetic force. If we imagine

the current in the conductor to be instantaneously reversed in

direction, the magnetic force surrounding it would not be instantly

reversed everywhere in direction, but the reversal would be propagated

outwards through space with a certain velocity which Maxwell showed was

inversely as the square root of the product of the magnetic permeability

and the dielectric constant or specific inductive capacity of the

medium.

These great results were announced by him for the first time in a paper

presented in 1864 to the Royal Society of London and printed in the

_Phil. Trans._ for 1865, entitled "A Dynamical Theory of the

Electromagnetic Field." Maxwell showed in this paper that the velocity

of propagation of an electromagnetic impulse through space could also be

determined by certain experimental methods which consisted in measuring

the same electric quantity, capacity, resistance or potential in two

ways. W.E. Weber had already laid the foundations of the absolute system

of electric and magnetic measurement, and proved that a quantity of

electricity could be measured either by the force it exercises upon

another static or stationary quantity of electricity, or magnetically by

the force this quantity of electricity exercises upon a magnetic pole

when flowing through a neighbouring conductor. The two systems of

measurement were called respectively the electrostatic and the

electromagnetic systems (see UNITS, PHYSICAL). Maxwell suggested new

methods for the determination of this ratio of the electrostatic to the

electromagnetic units, and by experiments of great ingenuity was able to

show that this ratio, which is also that of the velocity of the

propagation of an electromagnetic impulse through space, is identical

with that of light. This great fact once ascertained, it became clear

that the notion that electric phenomena are affections of the

luminiferous ether was no longer a mere speculation but a scientific

theory capable of verification. An immediate deduction from Maxwell's

theory was that in transparent dielectrics, the dielectric constant or

specific inductive capacity should be numerically equal to the square of

the refractive index for very long electric waves. At the time when

Maxwell developed his theory the dielectric constants of only a few

transparent insulators were known and these were for the most part

measured with steady or unidirectional electromotive force. The only

refractive indices which had been measured were the optical refractive

indices of a number of transparent substances. Maxwell made a comparison

between the optical refractive index and the dielectric constant of

paraffin wax, and the approximation between the numerical values of the

square of the first and that of the last was sufficient to show that

there was a basis for further work. Maxwell's electric and magnetic

ideas were gathered together in a great mathematical treatise on

electricity and magnetism which was published in 1873.[15] This book

stimulated in a most remarkable degree theoretical and practical

research into the phenomena of electricity and magnetism. Experimental

methods were devised for the further exact measurements of the

electromagnetic velocity and numerous determinations of the dielectric

constants of various solids, liquids and gases, and comparisons of these

with the corresponding optical refractive indices were conducted. This

early work indicated that whilst there were a number of cases in which

the square of optical refractive index for long waves and the

dielectric constant of the same substance were sufficiently close to

afford an apparent confirmation of Maxwell's theory, yet in other cases

there were considerable divergencies. L. Boltzmann (1844-1907) made a

large number of determinations for solids and for gases, and the

dielectric constants of many solid and liquid substances were determined

by N.N. Schiller (b. 1848), P.A. Silow (b. 1850), J. Hopkinson and

others. The accumulating determinations of the numerical value of the

electromagnetic velocity (v) from the earliest made by Lord Kelvin (Sir

W. Thomson) with the aid of King and M^cKichan, or those of Clerk

Maxwell, W.E. Ayrton and J. Perry, to more recent ones by J.J. Thomson,

F. Himstedt, H.A. Rowland, E.B. Rosa, J.S.H. Pellat and H.A. Abraham,

showed it to be very close to the best determinations of the velocity of

light (see UNITS, PHYSICAL). On the other hand, the divergence in some

cases between the square of the optical refractive index and the

dielectric constant was very marked. Hence although Maxwell's theory of

electrical action when first propounded found many adherents in Great

Britain, it did not so much dominate opinion on the continent of Europe.

FOURTH PERIOD.--With the publication of Clerk Maxwell's treatise in

1873, we enter fully upon the fourth and modern period of electrical

research. On the technical side the invention of a new form of armature

for dynamo electric machines by Z.T. Gramme (1826-1901) inaugurated a

departure from which we may date modern electrical engineering. It will

be convenient to deal with technical development first.

_Technical Development._--As far back as 1841 large magneto-electric

machines driven by steam power had been constructed, and in 1856 F.H.

Holmes had made a magneto machine with multiple permanent magnets which

was installed in 1862 in Dungeness lighthouse. Further progress was made

in 1867 when H. Wilde introduced the use of electromagnets for the field

magnets. In 1860 Dr Antonio Pacinotti invented what is now called the

toothed ring winding for armatures and described it in an Italian

journal, but it attracted little notice until reinvented in 1870 by

Gramme. In this new form of bobbin, the armature consisted of a ring of

iron wire wound over with an endless coil of wire and connected to a

commutator consisting of copper bars insulated from one another. Gramme

dynamos were then soon made on the self-exciting principle. In 1873 at

Vienna the fact was discovered that a dynamo machine of the Gramme type

could also act as an electric motor and was set in rotation when a

current was passed into it from another similar machine. Henceforth the

electric transmission of power came within the possibilities of

engineering.

_Electric Lighting._--In 1876, Paul Jablochkov (1847-1894), a Russian

officer, passing through Paris, invented his famous electric candle,

consisting of two rods of carbon placed side by side and separated from

one another by an insulating material. This invention in conjunction

with an alternating current dynamo provided a new and simple form of

electric arc lighting. Two years afterwards C.F. Brush, in the United

States, produced another efficient form of dynamo and electric arc lamp

suitable for working in series (see LIGHTING: _Electric_), and these

inventions of Brush and Jablochkov inaugurated commercial arc lighting.

The so-called subdivision of electric light by incandescent lighting

lamps then engaged attention. E.A. King in 1845 and W.E. Staite in 1848

had made incandescent electric lamps of an elementary form, and T.A.

Edison in 1878 again attacked the problem of producing light by the

incandescence of platinum. It had by that time become clear that the

most suitable material for an incandescent lamp was carbon contained in

a good vacuum, and St G. Lane Fox and Sir J.W. Swan in England, and T.A.

Edison in the United States, were engaged in struggling with the

difficulties of producing a suitable carbon incandescence electric lamp.

Edison constructed in 1879 a successful lamp of this type consisting of

a vessel wholly of glass containing a carbon filament made by

carbonizing paper or some other carbonizable material, the vessel being

exhausted and the current led into the filament through platinum wires.

In 1879 and 1880, Edison in the United States, and Swan in conjunction

with C.H. Stearn in England, succeeded in completely solving the

practical problems. From and after that date incandescent electric

lighting became commercially possible, and was brought to public notice

chiefly by an electrical exhibition held at the Crystal Palace, near

London, in 1882. Edison, moreover, as well as Lane-Fox, had realized the

idea of a public electric supply station, and the former proceeded to

establish in Pearl Street, New York, in 1881, the first public electric

supply station. A similar station in England was opened in the basement

of a house in Holborn Viaduct, London, in March 1882. Edison, with

copious ingenuity, devised electric meters, electric mains, lamp

fittings and generators complete for the purpose. In 1881 C.A. Faure

made an important improvement in the lead secondary battery which G.

Plante (1834-1889) had invented in 1859, and storage batteries then

began to be developed as commercial appliances by Faure, Swan, J.S.

Sellon and many others (see ACCUMULATOR). In 1882, numerous electric

lighting companies were formed for the conduct of public and private

lighting, but an electric lighting act passed in that year greatly

hindered commercial progress in Great Britain. Nevertheless the delay

was utilized in the completion of inventions necessary for the safe and

economical distribution of electric current for the purpose of electric

lighting.

_Telephone._--Going back a few years we find the technical applications

of electrical invention had developed themselves in other directions.

Alexander Graham Bell in 1876 invented the speaking telephone (q.v.),

and Edison and Elisha Gray in the United States followed almost

immediately with other telephonic inventions for electrically

transmitting speech. About the same time D.E. Hughes in England invented

the microphone. In 1879 telephone exchanges began to be developed in the

United States, Great Britain and other countries.

_Electric Power._--Following on the discovery in 1873 of the reversible

action of the dynamo and its use as a motor, efforts began to be made to

apply this knowledge to transmission of power, and S.D. Field, T.A.

Edison, Leo Daft, E.M. Bentley and W.H. Knight, F.J. Sprague, C.J. Van

Depoele and others between 1880 and 1884 were the pioneers of electric

traction. One of the earliest electric tram cars was exhibited by E.W.

and W. Siemens in Paris in 1881. In 1883 Lucien Gaulard, following a

line of thought opened by Jablochkov, proposed to employ high pressure

alternating currents for electric distributions over wide areas by means

of transformers. His ideas were improved by Carl Zipernowsky and O.T.

Blathy in Hungary and by S.Z. de Ferranti in England, and the

alternating current transformer (see TRANSFORMERS) came into existence.

Polyphase alternators were first exhibited at the Frankfort electrical

exhibition in 1891, developed as a consequence of scientific researches

by Galileo Ferraris (1847-1897), Nikola Tesla, M.O. von

Dolivo-Dobrowolsky and C.E.L. Brown, and long distance transmission of

electrical power by polyphase electrical currents (see POWER

TRANSMISSION: _Electric_) was exhibited in operation at Frankfort in

1891. Meanwhile the early continuous current dynamos devised by Gramme,

Siemens and others had been vastly improved in scientific principle and

practical construction by the labours of Siemens, J. Hopkinson, R.E.B.

Crompton, Elihu Thomson, Rudolf Eickemeyer, Thomas Parker and others,

and the theory of the action of the dynamo had been closely studied by

J. and E. Hopkinson, G. Kapp, S.P. Thompson, C.P. Steinmetz and J.

Swinburne, and great improvements made in the alternating current dynamo

by W.M. Mordey, S.Z. de Ferranti and Messrs Ganz of Budapest. Thus in

twenty years from the invention of the Gramme dynamo, electrical

engineering had developed from small beginnings into a vast industry.

The amendment, in 1888, of the Electric Lighting Act of 1882, before

long caused a huge development of public electric lighting in Great

Britain. By the end of the 19th century every large city in Europe and

in North and South America was provided with a public electric supply

for the purposes of electric lighting. The various improvements in

electric illuminants, such as the Nernst oxide lamp, the tantalum and

osmium incandescent lamps, and improved forms of arc lamp, enclosed,

inverted and flame arcs, are described under LIGHTING: _Electric_.

Between 1890 and 1900, electric traction advanced rapidly in the United

States of America but more slowly in England. In 1902 the success of

deep tube electric railways in Great Britain was assured, and in 1904

main line railways began to abandon, at least experimentally, the steam

locomotive and substitute for it the electric transmission of power.

Long distance electrical transmission had been before that time

exemplified in the great scheme of utilizing the falls of Niagara. The

first projects were discussed in 1891 and 1892 and completed practically

some ten years later. In this scheme large turbines were placed at the

bottom of hydraulic fall tubes 150 ft. deep, the turbines being coupled

by long shafts with 5000 H.P. alternating current dynamos on the

surface. By these electric current was generated and transmitted to

towns and factories around, being sent overhead as far as Buffalo, a

distance of 18 m. At the end of the 19th century electrochemical

industries began to be developed which depended on the possession of

cheap electric energy. The production of aluminium in Switzerland and

Scotland, carborundum and calcium carbide in the United States, and soda

by the Castner-Kellner process, began to be conducted on an immense

scale. The early work of Sir W. Siemens on the electric furnace was

continued and greatly extended by Henri Moissan and others on its

scientific side, and electrochemistry took its place as one of the most

promising departments of technical research and invention. It was

stimulated and assisted by improvements in the construction of large

dynamos and increased knowledge concerning the control of powerful

electric currents.

In the early part of the 20th century the distribution in bulk of

electric energy for power purposes in Great Britain began to assume

important proportions. It was seen to be uneconomical for each city and

town to manufacture its own supply since, owing to the intermittent

nature of the demand for current for lighting, the price had to be kept

up to 4d. and 6d. per unit. It was found that by the manufacture in

bulk, even by steam engines, at primary centres the cost could be

considerably reduced, and in numerous districts in England large power

stations began to be erected between 1903 and 1905 for the supply of

current for power purposes. This involved almost a revolution in the

nature of the tools used, and in the methods of working, and may

ultimately even greatly affect the factory system and the concentration

of population in large towns which was brought about in the early part

of the 19th century by the invention of the steam engine.

_Development of Electric Theory._

Turning now to the theory of electricity, we may note the equally

remarkable progress made in 300 years in scientific insight into the

nature of the agency which has so recast the face of human society.

There is no need to dwell upon the early crude theories of the action of

amber and lodestone. In a true scientific sense no hypothesis was

possible, because few facts had been accumulated. The discoveries of

Stephen Gray and C.F. de C. du Fay on the conductivity of some bodies

for the electric agency and the dual character of electrification gave

rise to the first notions of electricity as an imponderable fluid, or

non-gravitative subtile matter, of a more refined and penetrating kind

than ordinary liquids and gases. Its duplex character, and the fact that

the electricity produced by rubbing glass and vitreous substances was

different from that produced by rubbing sealing-wax and resinous

substances, seemed to necessitate the assumption of two kinds of

electric fluid; hence there arose the conception of _positive_ and

_negative_ electricity, and the two-fluid theory came into existence.

_Single-fluid Theory._--The study of the phenomena of the Leyden jar and

of the fact that the inside and outside coatings possessed opposite

electricities, so that in charging the jar as much positive electricity

is added to one side as negative to the other, led Franklin about 1750

to suggest a modification called the single fluid theory, in which the

two states of electrification were regarded as not the results of two

entirely different fluids but of the addition or subtraction of one

electric fluid from matter, so that positive electrification was to be

looked upon as the result of increase or addition of something to

ordinary matter and negative as a subtraction. The positive and negative

electrifications of the two coatings of the Leyden jar were therefore to

be regarded as the result of a transformation of something called

electricity from one coating to the other, by which process a certain

measurable quantity became so much less on one side by the same amount

by which it became more on the other. A modification of this single

fluid theory was put forward by F.U.T. Aepinus which was explained and

illustrated in his _Tentamen theoriae electricitatis et magnetismi_,

published in St Petersburg in 1759. This theory was founded on the

following principles:--(1) the particles of the electric fluid repel

each other with a force decreasing as the distance increases; (2) the

particles of the electric fluid attract the atoms of all bodies and are

attracted by them with a force obeying the same law; (3) the electric

fluid exists in the pores of all bodies, and while it moves without any

obstruction in conductors such as metals, water, &c., it moves with

extreme difficulty in so-called non-conductors such as glass, resin,

&c.; (4) electrical phenomena are produced either by the transference of

the electric fluid of a body containing more to one containing less, or

from its attraction and repulsion when no transference takes place.

Electric attractions and repulsions were, however, regarded as

differential actions in which the mutual repulsion of the particles of

electricity operated, so to speak, in antagonism to the mutual

attraction of particles of matter for one another and of particles of

electricity for matter. Independently of Aepinus, Henry Cavendish put

forward a single-fluid theory of electricity (_Phil. Trans._, 1771, 61,

p. 584), in which he considered it in more precise detail.

_Two-fluid Theory._--In the elucidation of electrical phenomena,

however, towards the end of the 18th century, a modification of the

two-fluid theory seems to have been generally preferred. The notion then

formed of the nature of electrification was something as follows:--All

bodies were assumed to contain a certain quantity of a so-called neutral

fluid made up of equal quantities of positive and negative electricity,

which when in this state of combination neutralized one another's

properties. The neutral fluid could, however, be divided up or separated

into its two constituents, and these could be accumulated on separate

conductors or non-conductors. This view followed from the discovery of

the facts of electric induction of J. Canton (1753, 1754). When, for

instance, a positively electrified body was found to induce upon another

insulated conductor a charge of negative electricity on the side nearest

to it, and a charge of positive electricity on the side farthest from

it, this was explained by saying that the particles of each of the two

electric fluids repelled one another but attracted those of the positive

fluid. Hence the operation of the positive charge upon the neutral fluid

was to draw towards the positive the negative constituent of the neutral

charge and repel to the distant parts of the conductor the positive

constituent.

C.A. Coulomb experimentally proved that the law of attraction and

repulsion of simple electrified bodies was that the force between them

varied inversely as the square of the distance and thus gave

mathematical definiteness to the two-fluid hypothesis. It was then

assumed that each of the two constituents of the neutral fluid had an

atomic structure and that the so-called particles of one of the electric

fluids, say positive, repelled similar particles with a force varying

inversely as a square of the distance and attracted those of the

opposite fluid according to the same law. This fact and hypothesis

brought electrical phenomena within the domain of mathematical analysis

and, as already mentioned, Laplace, Biot, Poisson, G.A.A. Plana

(1781-1846), and later Robert Murphy (1806-1843), made them the subject

of their investigations on the mode in which electricity distributes

itself on conductors when in equilibrium.

_Faraday's Views._--The two-fluid theory may be said to have held the

field until the time when Faraday began his researches on electricity.

After he had educated himself by the study of the phenomena of lines of

magnetic force in his discoveries on electromagnetic induction, he

applied the same conception to electrostatic phenomena, and thus created

the notion of lines of electrostatic force and of the important function

of the dielectric or non-conductor in sustaining them. Faraday's notion

as to the nature of electrification, therefore, about the middle of the

19th century came to be something as follows:--He considered that the

so-called charge of electricity on a conductor was in reality nothing on

the conductor or in the conductor itself, but consisted in a state of

strain or polarization, or a physical change of some kind in the

particles of the dielectric surrounding the conductor, and that it was

this physical state in the dielectric which constituted electrification.

Since Faraday was well aware that even a good vacuum can act as a

dielectric, he recognized that the state he called dielectric

polarization could not be wholly dependent upon the presence of

gravitative matter, but that there must be an electromagnetic medium of

a supermaterial nature. In the 13th series of his _Experimental

Researches on Electricity_ he discussed the relation of a vacuum to

electricity. Furthermore his electrochemical investigations, and

particularly his discovery of the important law of electrolysis, that

the movement of a certain quantity of electricity through an electrolyte

is always accompanied by the transfer of a certain definite quantity of

matter from one electrode to another and the liberation at these

electrodes of an equivalent weight of the ions, gave foundation for the

idea of a definite atomic charge of electricity. In fact, long

previously to Faraday's electrochemical researches, Sir H. Davy and J.J.

Berzelius early in the 19th century had advanced the hypothesis that

chemical combination was due to electric attractions between the

electric charges carried by chemical atoms. The notion, however, that

electricity is atomic in structure was definitely put forward by Hermann

von Helmholtz in a well-known Faraday lecture. Helmholtz says: "If we

accept the hypothesis that elementary substances are composed of atoms,

we cannot well avoid concluding that electricity also is divided into

elementary portions which behave like atoms of electricity."[16] Clerk

Maxwell had already used in 1873 the phrase, "a molecule of

electricity."[17] Towards the end of the third quarter of the 19th

century it therefore became clear that electricity, whatever be its

nature, was associated with atoms of matter in the form of exact

multiples of an indivisible minimum electric charge which may be

considered to be "Nature's unit of electricity." This ultimate unit of

electric quantity Professor Johnstone Stoney called an _electron_.[18]

The formulation of electrical theory as far as regards operations in

space free from matter was immensely assisted by Maxwell's mathematical

theory. Oliver Heaviside after 1880 rendered much assistance by reducing

Maxwell's mathematical analysis to more compact form and by introducing

greater precision into terminology (see his _Electrical Papers_, 1892).

This is perhaps the place to refer also to the great services of Lord

Rayleigh to electrical science. Succeeding Maxwell as Cavendish

professor of physics at Cambridge in 1880, he soon devoted himself

especially to the exact redetermination of the practical electrical

units in absolute measure. He followed up the early work of the British

Association Committee on electrical units by a fresh determination of

the ohm in absolute measure, and in conjunction with other work on the

electrochemical equivalent of silver and the absolute electromotive

force of the Clark cell may be said to have placed exact electrical

measurement on a new basis. He also made great additions to the theory

of alternating electric currents, and provided fresh appliances for

other electrical measurements (see his _Collected Scientific Papers_,

Cambridge, 1900).

_Electro-optics._--For a long time Faraday's observation on the rotation

of the plane of polarized light by heavy glass in a magnetic field

remained an isolated fact in electro-optics. Then M.E. Verdet

(1824-1860) made a study of the subject and discovered that a solution

of ferric perchloride in methyl alcohol rotated the plane of

polarization in an opposite direction to heavy glass (_Ann. Chim.

Phys._, 1854, 41, p. 370; 1855, 43, p. 37; _Com. Rend._, 1854, 39, p.

548). Later A.A.E.E. Kundt prepared metallic films of iron, nickel and

cobalt, and obtained powerful negative optical rotation with them

(_Wied. Ann._, 1884, 23, p. 228; 1886, 27, p. 191). John Kerr

(1824-1907) discovered that a similar effect was produced when plane

polarized light was reflected from the pole of a powerful magnet (_Phil.

Mag._, 1877, [5], 3, p. 321, and 1878, 5, p. 161). Lord Kelvin showed

that Faraday's discovery demonstrated that some form of rotation was

taking place along lines of magnetic force when passing through a

medium.[19] Many observers have given attention to the exact

determination of Verdet's constant of rotation for standard substances,

e.g. Lord Rayleigh for carbon bisulphide,[20] and Sir W.H. Perkin for an

immense range of inorganic and organic bodies.[21] Kerr also discovered

that when certain homogeneous dielectrics were submitted to electric

strain, they became birefringent (_Phil. Mag._, 1875, 50, pp. 337 and

446). The theory of electro-optics received great attention from Kelvin,

Maxwell, Rayleigh, G.F. Fitzgerald, A. Righi and P.K.L. Drude, and

experimental contributions from innumerable workers, such as F.T.

Trouton, O.J. Lodge and J.L. Howard, and many others.

_Electric Waves._--In the decade 1880-1890, the most important advance

in electrical physics was, however, that which originated with the

astonishing researches of Heinrich Rudolf Hertz (1857-1894). This

illustrious investigator was stimulated, by a certain problem brought to

his notice by H. von Helmholtz, to undertake investigations which had

for their object a demonstration of the truth of Maxwell's principle

that a variation in electric displacement was in fact an electric

current and had magnetic effects. It is impossible to describe here the

details of these elaborate experiments; the reader must be referred to

Hertz's own papers, or the English translation of them by Prof. D.E.

Jones. Hertz's great discovery was an experimental realization of a

suggestion made by G.F. Fitzgerald (1851-1901) in 1883 as to a method of

producing electric waves in space. He invented for this purpose a

radiator consisting of two metal rods placed in one line, their inner

ends being provided with poles nearly touching and their outer ends with

metal plates. Such an arrangement constitutes in effect a condenser, and

when the two plates respectively are connected to the secondary

terminals of an induction coil in operation, the plates are rapidly and

alternately charged, and discharged across the spark gap with electrical

oscillations (see ELECTROKINETICS). Hertz then devised a wave detecting

apparatus called a resonator. This in its simplest form consisted of a

ring of wire nearly closed terminating in spark balls very close

together, adjustable as to distance by a micrometer screw. He found that

when the resonator was placed in certain positions with regard to the

oscillator, small sparks were seen between the micrometer balls, and

when the oscillator was placed at one end of a room having a sheet of

zinc fixed against the wall at the other end, symmetrical positions

could be found in the room at which, when the resonator was there

placed, either no sparks or else very bright sparks occurred at the

poles. These effects, as Hertz showed, indicated the establishment of

stationary electric waves in space and the propagation of electric and

magnetic force through space with a finite velocity. The other

additional phenomena he observed finally contributed an all but

conclusive proof of the truth of Maxwell's views. By profoundly

ingenious methods Hertz showed that these invisible electric waves could

be reflected and refracted like waves of light by mirrors and prisms,

and that familiar experiments in optics could be repeated with electric

waves which could not affect the eye. Hence there arose a new science of

electro-optics, and in all parts of Europe and the United States

innumerable investigators took possession of the novel field of research

with the greatest delight. O.J. Lodge,[22] A. Righi,[23] J.H.

Poincare,[24] V.F.K. Bjerknes, P.K.L. Drude, J.J. Thomson,[25] John

Trowbridge, Max Abraham, and many others, contributed to its

elucidation.

In 1892, E. Branly of Paris devised an appliance for detecting these

waves which subsequently proved to be of immense importance. He

discovered that they had the power of affecting the electric

conductivity of materials when in a state of powder, the majority of

metallic filings increasing in conductivity. Lodge devised a similar

arrangement called a coherer, and E. Rutherford invented a magnetic

detector depending on the power of electric oscillations to demagnetize

iron or steel. The sum total of all these contributions to electrical

knowledge had the effect of establishing Maxwell's principles on a firm

basis, but they also led to technical inventions of the very greatest

utility. In 1896 G. Marconi applied a modified and improved form of

Branly's wave detector in conjunction with a novel form of radiator for

the telegraphic transmission of intelligence through space without

wires, and he and others developed this new form of telegraphy with the

greatest rapidity and success into a startling and most useful means of

communicating through space electrically without connecting wires.

_Electrolysis._--The study of the transfer of electricity through

liquids had meanwhile received much attention. The general facts and

laws of electrolysis (q.v.) were determined experimentally by Davy and

Faraday and confirmed by the researches of J.F. Daniell, R.W. Bunsen and

Helmholtz. The modern theory of electrolysis grew up under the hands of

R.J.E. Clausius, A.W. Williamson and F.W.G. Kohlrausch, and received a

great impetus from the work of Svante Arrhenius, J.H. Van't Hoff, W.

Ostwald, H.W. Nernst and many others. The theory of the ionization of

salts in solution has raised much discussion amongst chemists, but the

general fact is certain that electricity only moves through liquids in

association with matter, and simultaneously involves chemical

dissociation of molecular groups.

_Discharge through Gases._--Many eminent physicists had an instinctive

feeling that the study of the passage of electricity through gases would

shed much light on the intrinsic nature of electricity. Faraday devoted

to a careful examination of the phenomena the XIII^th series of his

_Experimental Researches_, and among the older workers in this field

must be particularly mentioned J. Plucker, J.W. Hittorf, A.A. de la

Rive, J.P. Gassiot, C.F. Varley, and W. Spottiswoode and J. Fletcher

Moulton. It has long been known that air and other gases at the pressure

of the atmosphere were very perfect insulators, but that when they were

rarefied and contained in glass tubes with platinum electrodes sealed

through the glass, electricity could be passed through them under

sufficient electromotive force and produced a luminous appearance known

as the electric glow discharge. The so-called vacuum tubes constructed

by H. Geissler (1815-1879) containing air, carbonic acid, hydrogen, &c.,

under a pressure of one or two millimetres, exhibit beautiful

appearances when traversed by the high tension current produced by the

secondary circuit of an induction coil. Faraday discovered the existence

of a dark space round the negative electrode which is usually known as

the "Faraday dark space." De la Rive added much to our knowledge of the

subject, and J. Plucker and his disciple J.W. Hittorf examined the

phenomena exhibited in so-called high vacua, that is, in exceedingly

rarefied gases. C.F. Varley discovered the interesting fact that no

current could be sent through the rarefied gas unless a certain minimum

potential difference of the electrodes was excited. Sir William Crookes

took up in 1872 the study of electric discharge through high vacua,

having been led to it by his researches on the radiometer. The

particular details of the phenomena observed will be found described in

the article CONDUCTION, ELECTRIC (S III.). The main fact discovered by

researches of Plucker, Hittorf and Crookes was that in a vacuum tube

containing extremely rarefied air or other gas, a luminous discharge

takes place from the negative electrode which proceeds in lines normal

to the surface of the negative electrode and renders phosphorescent both

the glass envelope and other objects placed in the vacuum tube when it

falls upon them. Hittorf made in 1869 the discovery that solid objects

could cast shadows or intercept this cathode discharge. The cathode

discharge henceforth engaged the attention of many physicists. Varley

had advanced tentatively the hypothesis that it consisted in an actual

projection of electrified matter from the cathode, and Crookes was led

by his researches in 1870, 1871 and 1872 to embrace and confirm this

hypothesis in a modified form and announce the existence of a fourth

state of matter, which he called radiant matter, demonstrating by many

beautiful and convincing experiments that there was an actual projection

of material substance of some kind possessing inertia from the surface

of the cathode. German physicists such as E. Goldstein were inclined to

take another view. Sir J.J. Thomson, the successor of Maxwell and Lord

Rayleigh in the Cavendish chair of physics in the university of

Cambridge, began about the year 1899 a remarkable series of

investigations on the cathode discharge, which finally enabled him to

make a measurement of the ratio of the electric charge to the mass of

the particles of matter projected from the cathode, and to show that

this electric charge was identical with the atomic electric charge

carried by a hydrogen ion in the act of electrolysis, but that the mass

of the cathode particles, or "corpuscles" as he called them, was far

less, viz. about 1/2000th part of the mass of a hydrogen atom.[26] The

subject was pursued by Thomson and the Cambridge physicists with great

mathematical and experimental ability, and finally the conclusion was

reached that in a high vacuum tube the electric charge is carried by

particles which have a mass only a fraction, as above mentioned, of that

of the hydrogen atom, but which carry a charge equal to the unit

electric charge of the hydrogen ion as found by electrochemical

researches.[27] P.E.A. Lenard made in 1894 (_Wied. Ann. Phys._, 51, p.

225) the discovery that these cathode particles or corpuscles could pass

through a window of thin sheet aluminium placed in the wall of the

vacuum tube and give rise to a class of radiation called the Lenard

rays. W.C. Rontgen of Munich made in 1896 his remarkable discovery of

the so-called X or Rontgen rays, a class of radiation produced by the

impact of the cathode particles against an impervious metallic screen or

anticathode placed in the vacuum tube. The study of Rontgen rays was

ardently pursued by the principal physicists in Europe during the years

1897 and 1898 and subsequently. The principal property of these Rontgen

rays which attracted public attention was their power of passing through

many solid bodies and affecting a photographic plate. Hence some

substances were opaque to them and others transparent. The astonishing

feat of photographing the bones of the living animal within the tissues

soon rendered the Rontgen rays indispensable in surgery and directed an

army of investigators to their study.

_Radioactivity._--One outcome of all this was the discovery by H.

Becquerel in 1896 that minerals containing uranium, and particularly the

mineral known as pitchblende, had the power of affecting sensitive

photographic plates enclosed in a black paper envelope when the mineral

was placed on the outside, as well as of discharging a charged

electroscope (_Com. Rend._, 1896, 122, p. 420). This research opened a

way of approach to the phenomena of radioactivity, and the history of

the steps by which P. Curie and Madame Curie were finally led to the

discovery of radium is one of the most fascinating chapters in the

history of science. The study of radium and radioactivity (see

RADIOACTIVITY) led before long to the further remarkable knowledge that

these so-called radioactive materials project into surrounding space

particles or corpuscles, some of which are identical with those

projected from the cathode in a high vacuum tube, together with others

of a different nature. The study of radioactivity was pursued with great

ability not only by the Curies and A. Debierne, who associated himself

with them, in France, but by E. Rutherford and F. Soddy in Canada, and

by J.J. Thomson, Sir William Crookes, Sir William Ramsay and others in

England.

_Electronic Theory._--The final outcome of these investigations was the

hypothesis that Thomson's corpuscles or particles composing the cathode

discharge in a high vacuum tube must be looked upon as the ultimate

constituent of what we call negative electricity; in other words, they

are atoms of negative electricity, possessing, however, inertia, and

these negative electrons are components at any rate of the chemical

atom. Each electron is a point-charge of negative electricity equal to

3.9 X 10^(-10) of an electrostatic unit or to 1.3 X 10^(-20) of an

electromagnetic unit, and the ratio of its charge to its mass is nearly

2 X 10^7 using E.M. units. For the hydrogen atom the ratio of charge to

mass as deduced from electrolysis is about 10^4. Hence the mass of an

electron is 1/2000th of that of a hydrogen atom. No one has yet been

able to isolate positive electrons, or to give a complete demonstration

that the whole inertia of matter is only electric inertia due to what

may be called the inductance of the electrons. Prof. Sir J. Larmor

developed in a series of very able papers (_Phil. Trans._, 1894, 185;

1895, 186; 1897, 190), and subsequently in his book _Aether and Matter_

(1900), a remarkable hypothesis of the structure of the electron or

corpuscle, which he regards as simply a strain centre in the aether or

electromagnetic medium, a chemical atom being a collection of positive

and negative electrons or strain centres in stable orbital motion round

their common centre of mass (see AETHER). J.J. Thomson also developed

this hypothesis in a profoundly interesting manner, and we may therefore

summarize very briefly the views held on the nature of electricity and

matter at the beginning of the 20th century by saying that the term

electricity had come to be regarded, in part at least, as a collective

name for electrons, which in turn must be considered as constituents of

the chemical atom, furthermore as centres of certain lines of

self-locked and permanent strain existing in the universal aether or

electromagnetic medium. Atoms of matter are composed of congeries of

electrons and the inertia of matter is probably therefore only the

inertia of the electromagnetic medium.[28] Electric waves are produced

wherever electrons are accelerated or retarded, that is, whenever the

velocity of an electron is changed or accelerated positively or

negatively. In every solid body there is a continual atomic

dissociation, the result of which is that mixed up with the atoms of

chemical matter composing them we have a greater or less percentage of

free electrons. The operation called an electric current consists in a

diffusion or movement of these electrons through matter, and this is

controlled by laws of diffusion which are similar to those of the

diffusion of liquids or gases. Electromotive force is due to a

difference in the density of the electronic population in different or

identical conducting bodies, and whilst the electrons can move freely

through so-called conductors their motion is much more hindered or

restricted in non-conductors. Electric charge consists, therefore, in an

excess or deficit of negative electrons in a body. In the hands of H.A.

Lorentz, P.K.L. Drude, J. J, Thomson, J. Larmor and many others, the

electronic hypothesis of matter and of electricity has been developed in

great detail and may be said to represent the outcome of modern

researches upon electrical phenomena.

The reader may be referred for an admirable summary of the theories of

electricity prior to the advent of the electronic hypothesis to J.J.

Thomson's "Report on Electrical Theories" (_Brit. Assoc. Report_, 1885),

in which he divides electrical theories enunciated during the 19th

century into four classes, and summarizes the opinions and theories of

A.M. Ampere, H.G. Grassman, C.F. Gauss, W.E. Weber, G.F.B. Riemann,

R.J.E. Clausius, F.E. Neumann and H. von Helmholtz.

BIBLIOGRAPHY.--M. Faraday, _Experimental Researches in Electricity_ (3

vols., London, 1839, 1844, 1855); A.A. De la Rive, _Treatise on

Electricity_ (3 vols., London, 1853, 1858); J. Clerk Maxwell, _A

Treatise on Electricity and Magnetism_ (2 vols., 3rd ed., 1892); id.,

_Scientific Papers_ (2 vols., edited by Sir W.J. Niven, Cambridge,

1890); H.M. Noad, _A Manual of Electricity_ (2 vols., London, 1855,

1857); J.J. Thomson, _Recent Researches in Electricity and Magnetism_

(Oxford, 1893); id., _Conduction of Electricity through Gases_

(Cambridge, 1903); id., _Electricity and Matter_ (London, 1904); O.

Heaviside, _Electromagnetic Theory_ (London, 1893); O.J. Lodge,

_Modern Views of Electricity_ (London, 1889); E. Mascart and J.

Joubert, _A Treatise on Electricity and Magnetism_, English trans. by

E. Atkinson (2 vols., London, 1883); Park Benjamin, _The Intellectual

Rise in Electricity_ (London, 1895); G.C. Foster and A.W. Porter,

_Electricity and Magnetism_ (London, 1903); A. Gray, _A Treatise on

Magnetism and Electricity_ (London, 1898); H.W. Watson and S.H.

Burbury, _The Mathematical Theory of Electricity and Magnetism_ (2

vols., 1885); Lord Kelvin (Sir William Thomson), _Mathematical and

Physical Papers_ (3 vols., Cambridge, 1882); Lord Rayleigh,

_Scientific Papers_ (4 vols., Cambridge, 1903); A. Winkelmann,

_Handbuch der Physik_, vols. iii. and iv. (Breslau, 1903 and 1905; a

mine of wealth for references to original papers on electricity and

magnetism from the earliest date up to modern times). For particular

information on the modern Electronic theory the reader may consult W.

Kaufmann, "The Developments of the Electron Idea." _Physikalische

Zeitschrift_ (1st of Oct. 1901), or _The Electrician_ (1901), 48, p.

95; H.A. Lorentz, _The Theory of Electrons_ (1909); E.E. Fournier

d'Albe, _The Electron Theory_ (London, 1906); H. Abraham and P.

Langevin, _Ions, Electrons, Corpuscles_ (Paris, 1905); J.A. Fleming,

"The Electronic Theory of Electricity," _Popular Science Monthly_ (May

1902); Sir Oliver J. Lodge, _Electrons, or the Nature and Properties

of Negative Electricity_ (London, 1907). (J. A. F.)