The Romans drew vast quantities of gold from the mines of Nubia (_nub_
means 'gold') and made Egypt their 'granary'. Egyptian religious beliefs
and customs were perpetuated by the Roman emperors. Tiberius and Vespasian
restored ancient Nilotic temples. The worship of Isis spread to Rome.
Hadrian had to give a decision in a dispute between Memphis and Heliopolis
regarding the sacred bull. But Egyptian native learning was decaying, and
the knowledge of hieroglyphic writing was dying out. Christianity was
introduced during the Roman period, and the Coptic Church established. In
A.D. 642 the Romans finally abandoned Egypt, which, till 868, became a
province of the successive Mohammedan caliphates of Medina, of Damascus,
and Baghdad. The Turkish soldiery dominated Egypt for a period. The Shia
heretics afterwards became powerful, and the Christians were well treated.
In 1250 the Mamelukes (descendants of slaves) came into power. Their
pomp-loving sultans and emirs lived in great splendour. They came under
Turkish sway early in the sixteenth century, but when Napoleon conquered
Egypt in the eighteenth century they were again semi-independent. The
British drove the French out of Egypt. Mehemet Ali, an Albanian officer in
a Turko-Albanian force, had himself declared Sultan of Egypt, but when he
overran Syria and threatened to march to Constantinople, Russia intervened.
Britain and France afterwards prevailed on Mehemet Ali to rule Egypt as the
viceroy of the Sultan of Turkey. His successor and grandson, Abbas I, built
the first railway in Egypt. The next viceroy, Said Pasha, son of Mehemet
Ali, granted to a French company the right to construct the Suez Canal.
Egypt became bankrupt under his successor, Ismail Pasha, the first Khedive
(Prince), during whose reign the Suez Canal was opened. He was deposed when
the British and French took over the control of Egyptian finance. During
the term of his successor, Tewfik Pasha, the Arabi Pasha rebellion took
place. The military occupation of Egypt by British troops was followed by
peace and good government. But trouble broke out in the Sudan. Mohammed
Ahmed declared himself the Mahdi (Messiah) of the Mohammedans, and
conquered a great part of the Sudan. In Nov., 1883, General Hicks ('Hicks
Pasha') led an army of 10,000 Egyptians against the false prophet, but
while marching across the driest part of the Sudan, misled by spies who
acted as guides, his thirst-stricken army was entirely destroyed by the
Mahdi's force. This victory gave the false prophet great prestige. In Jan.,
1884, General Gordon was sent to Khartoum as Governor-General of the Sudan,
but was completely isolated there. Khartoum was captured and the gallant
general slain on 26th Jan., 1885, before a relieving force could reach him.
The Mahdi died in June, 1885, and was succeeded by Abdullah the Khalifa.
After a period of reorganization and preparation in Egypt, the reconquest
of the Sudan was begun. Lord (then Sir Herbert) Kitchener was Sirdar, or
Commander-in-Chief of the Egyptian army, and his expeditionary force was
strengthened by British regiments. In April, 1898, the Khalifa's army was
defeated on the banks of the Atbara, and on 2nd Sept. Kitchener won a great
victory near Omdurman. The Khalifa escaped, but was rounded up by Sir
Reginald Wingate's force, and slain with his emirs at Umme Dubraykat on
24th Nov., 1899. Thereafter the Sudan came under the control of a
British-Egyptian _condominium_, which appointed a Governor-General.
[Illustration: The Great Pyramids of Giza and the Sphinx
Built by Cheops (Khufu) and Cephron (Khafra) as their future tombs, in
order to secure immortality by the preservation of the mummy.]
At the time of the outbreak of the Great War, in the autumn of 1914, the
Khedive of Egypt was in Constantinople. He sided with the Central Powers.
He was consequently deposed by Britain, and Prince Hussein Kamil was
declared Sultan of Egypt; the suzerainty of Turkey terminated at the same
time. The new ruler of Egypt was the uncle of the deposed Khedive, Abbas
Pasha Hilmi (second son of the first Khedive, Ismail Pasha, and brother of
Tewfik Pasha, the second Khedive). Hussein Kamil died in 1917 and was
succeeded by Ahmed Fuad Pasha. Under the Peace Treaty, Egypt is recognized
as an independent kingdom protected by Great Britain. The capital of modern
Egypt is Cairo, situated near the site of ancient Memphis. Thebes is
represented by Luxor and Karnak.--BIBLIOGRAPHY: (Religion) Breasted,
_Development of Religion and Thought in Ancient Egypt_; Renouf, _Book of
the Dead_; Budge, _Gods of the Egyptians_; Wiedemann, _Religion of the
Ancient Egyptians_; Sayce, _Religion of Ancient Egypt and Babylonia_; G.
Elliot Smith, _The Migrations of Early Culture_; (History)--G. Elliot
Smith, _The Ancient Egyptians_; Breasted, _A History of Egypt_; Flinders
Petrie, _A History of Egypt_; King and Hall, _Egypt and Western Asia in the
Light of Recent Discoveries_; H. R. Hall, _The Ancient History of the Near
East_; Sir A. Colvin, _The Making of Modern Egypt_; Lord Cromer, _Modern
Egypt_.
EGYPTIAN BLUE, a brilliant and very permanent pigment used by the Romans in
the early centuries of the Christian era. It has been found in ancient
frescoes in the Vatican, and also at Pompeii. The chemist Fouqu['e] proved
by analysis that it is a double silicate of calcium and copper.
EGYPTIAN VULTURE (_Neophron perenopt[)e]rus_), a bird that frequents both
shores of the Mediterranean, but rarely passes farther north, though it has
been found in the British islands. It is one of the smaller vultures, about
the size of a raven. The general colour is white, the quill feathers of the
wing being dark brown. It frequents the streets of Eastern towns, where it
is protected on account of its services as a scavenger. This vulture is
sometimes known as Pharaoh's Hen, on account of its frequent representation
in Egyptian hieroglyphics. In Spain it is called the _quebranta-huesos_
(bone-smasher) in reference to its supposed habit of breaking up bones left
by other vultures.
EHRENBERG ([=a]'r[.e]n-ber_h_), Christian Gottfried, a German scientist,
born in 1795, died in 1876. After studying theology, medicine, and natural
history, he joined in 1820 an expedition to Palestine, Egypt, and
Abyssinia, returning to Berlin in 1825. In 1829 he accompanied Humboldt to
the Ural and Altai ranges and to Central Siberia. His great work on
Infusoria appeared in 1838, and was at once recognized as the highest
authority on the subject. It was followed in 1854 by his _Microgeology_.
EHRENBREITSTEIN ([=a]'ren-br[=i]t-st[=i]n), a dismantled Prussian fortress
formerly of great strength and situated opposite the confluence of the
Moselle with the Rhine, on a precipitous rock 387 feet above the river, and
inaccessible on three sides. It is connected with Coblentz on the opposite
shore by a bridge of boats. The fortifications, which were erected between
1816 and 1820 at a cost of L1,200,000, could accommodate a garrison of
14,000 men, and possessed room for stores to last an army of 60,000 for a
year.
EHRLICH, Paul, German physician, born in Silesia in 1854, died in 1915.
Educated at Breslau, Strasburg, and Leipzig, he became _privat-dozent_ at
the University of Berlin in 1889, and in 1896 was appointed director of the
Royal Institute for Serum Research at Steglitz, which was transferred to
Frankfurt in 1899, and became the Royal Institute for Experimental
Therapeutics. His studies in the histology of blood are very important, but
his claim to fame is based upon his discovery of _salvarsan_ (606) and of
_neosalvarsan_ (614), arsenic compounds which are very efficacious in the
treatment of syphilis. He delivered the Croonian lectures in 1900 and the
Harben lectures in 1907, received honorary degrees from the universities of
Oxford and Chicago, and in 1908 shared the Nobel prize for medicine with
Metchnikoff of the Institut Pasteur in Paris. His works include:
_Beitr[:a]ge zu Histologie und Klinik des Blutes_, _Anaemie_, _Abhandlungen
[:u]ber Salvarsan_, &c.
EIBENSTOCK ([=i]'ben-stok), a town in the south-east of Saxony, with
important manufactures of lace. Pop. 9528.
EICHHORN ([=i]_h_'horn), Johann Gottfried, German Orientalist, historian,
&c., born in 1752, died in 1827. He became professor of Oriental languages
at Jena, and then at G[:o]ttingen. Amongst his works are: _The Hebrew
Prophets_, _History of Literature_, _History of the Last Three Centuries_,
_Introductions to the Old and New Testaments and to the Apocrypha_.
EICHST[:A]TT ([=i]_h_'stet), an old town, Bavaria, in a deep valley of the
Altm[:u]hl, 67 miles N.N.W. of Munich. Its principal edifice is a fine
Gothic cathedral, founded in 1259. Pop. 8029.
EIDER ([=i]'d[.e]r), a river of Schleswig-Holstein, rises 8 miles S. of
Kiel, and after a winding course falls into the North Sea at T[:o]nning;
length, 112 miles. By means of a canal it long gave communication between
the North Sea and Baltic, but the new ship canal here has superseded this
route.
[Illustration: Eider Duck (_Somateria molliss[)i]ma_), female]
EIDER DUCK (_Somateria molliss[)i]ma_), a species of duck found from 45deg
north to the highest latitudes yet visited, both in Europe and America. Its
favourite haunts are solitary rocky shores and islands. In Greenland and
Iceland they occur in great numbers, and also breed on the western islands
of Scotland. The eider duck is about twice the size of the common duck,
being about 2 feet 3 inches in length, 3 feet in breadth of wing, and from
6 to 7 lb. in weight. The male is black, head and back white, with a black
crown. The female is reddish drab spotted with black, and with two white
bands on the wings. They feed largely on shell-fish and crustaceans. Their
nests are usually formed of drift grass or dry seaweed, lined with a large
quantity of down, which the female plucks from her own breast. In this soft
bed she lays five eggs, which she covers over with a layer of down. If
this, with the eggs, is removed, the bird repeats the process. One female
generally furnishes about 1/2 lb. of down, but the quantity is reduced by
cleaning. This down, from its superior warmth, lightness, and elasticity,
is in great demand for beds and coverlets; and the districts in Norway and
Iceland where these birds abound are guarded with the greatest vigilance as
a most valuable property. As found in commerce this down is in balls of the
size of a man's fist, and weighing from 3 to 4 lb. It is so fine and
elastic that 5 lb. of the best quality is sufficient for a whole bed. The
down from dead birds is little esteemed, having lost its elasticity. The
king eider duck (_Somateria spectab['i]lis_) is another species resembling
the preceding and inhabiting the same coasts.--Cf. J. G. Millais, _British
Diving Ducks_.
EIFFEL ([=a]-fel), Alexandre Gustave, French engineer, born in 1832;
attended, from 1852 to 1855, the ['E]cole Centrale des Arts et Manufactures
at Paris, and devoted himself chiefly to the designing of large structures
in iron, especially bridges and viaducts, the great bridge over the Douro
being one of his works. His name is best known, however, from the lofty
iron tower erected by him in connection with the Paris Exhibition of 1889,
rising to the height of 985 feet on the Champ de Mars. He was condemned in
1893 to two years' imprisonment and a fine of 20,000 francs for
misappropriation of funds belonging to the Panama Canal Company, but the
judgment was set aside on technical grounds. In 1913 he published a work
entitled _Resistance of the Air_. See next article.
EIFFEL TOWER, a structure named after the originator, one of the sights of
Paris, is by far the loftiest structure in existence, surpassing the
Washington Obelisk, the next highest (555 feet), by 430 feet. It cost about
L260,000, and was erected partly at the cost of the State, partly by funds
provided by Eiffel himself, who formed a company for the purpose. The
company derived its profits from the fees which visitors had to pay, but
the tower became the property of the State in 1909. The top may be reached
by stairs and lifts. The first stage or platform is at the height of 189
feet, and forms a quadrilateral 213 feet square, fitted up as a restaurant.
The next platform is at the height of about 380 feet, and is 98 feet
square. The third platform is at the height of 906 feet, and is large
enough to accommodate a good number of persons, affording a magnificent
view. The lantern higher up is supplied with powerful electric
searchlights, and on the very summit is a small area utilized chiefly for
scientific observations. The tower has been utilized in experiments
connected with the fall of bodies, vibration of the pendulum, and pressure
of the air. In recent years the tower has become an important wireless
telegraphy station.
EIGG (eg), an island on the west coast of Scotland, county of Inverness,
about 10 miles from the mainland, and 5 miles long by about 3 broad. It has
bold, rocky shores, and terminates to the south in a lofty promontory
called the Scuir of Eigg, with a peak of columnar pitchstone porphyry 1339
feet above the sea, and on one side perpendicular as a wall. It is the
scene of the massacre, towards the end of the sixteenth century, of 200
Macdonalds by the Macleods of Skye, who suffocated them in a cave where
they had taken refuge. Pop. 211.
EIGHT-HOUR DAY. The eight-hour day was proposed in England as early as
1833, and in 1869 the Trade Union Congress of Birmingham formulated the
demand that this ideal working day should be adopted throughout the United
Kingdom. The Trade Unions and the Socialists sought to secure the
establishment of the eight-hour day through legislation, and it gradually
came into force not only in coal-mining, but in various trades and
industries. The agitation in favour of an eight-hour day became very strong
in Europe towards the end of the nineteenth and at the beginning of the
twentieth centuries, and in England it was granted to miners in 1908, and
to railway employees in 1919. Since the European War the movement has made
great headway, and it now forms part of the programme of the Labour parties
in almost all European countries.--BIBLIOGRAPHY: Hedfield and Gibbins, _A
Shorter Working Day_; Robertson, _The Eight Hour Question_.
EIGHTY CLUB, THE, a club formed in 1879 by a number of prominent English
Liberals with a view to the promotion of the success of Liberalism at the
general election of 1880, whence its name. The members lecture on political
subjects and address Liberal associations throughout the country. Women
were first admitted to membership in 1920.
EIKON BASILIK[=E] ([=i]'kon ba-sil'i-k[=e]; Gr., 'the royal image'), the
name of a book published shortly after the execution of Charles I in Jan.,
1649, and supposed by some to have been written by the king himself. At the
Restoration Gauden, afterwards Bishop of Worcester, laid claim to the
authorship, and a memorandum in the copy of the Earl of Anglesea, Lord
Privy Seal under Charles II, affirms his claim with the authority of
Charles II and the Duke of York. The Royalist Clarendon, author of the
_History of the Rebellion_, accepted this statement, but others refused to
credit Gauden with the authorship. Within a year of its publication, 48,000
copies of the book were sold, and the republicans put forward Milton to
answer it, his _Eikonoklastes_ (that is 'image-breaker') appearing the same
year, by order of Parliament. The _Eikon Basilik[=e]_ professes to be a
sort of private journal of the king, written in an affectedly dignified
strain, and containing numerous assertions of love for his misguided and
ungrateful people.--Cf. Almack, _Bibliography of the King's Book, or Eikon
Basilik[=e]_.
EILDON HILLS ([=e]l'don), a picturesque hill-mass with three summits, south
of Melrose, Roxburghshire, Scotland, reaching a height of nearly 1400 feet,
fabled to have been cleft in three by Michael Scott.
EILEITHYIA ([=i]-l[=i]-th[=i]'ya), the Greek name of the ancient Egyptian
city Nekheb (the modern El-Kab), on the Nile, some distance above Esneh.
Important remains have been obtained from rock-tombs in the neighbourhood,
and there are several ruined temples.
EILENBURG ([=i]'l[.e]n-b[u:]r_h_), a town, Prussian Saxony, 26 miles N.N.E.
of Merseburg, on an island of the Mulde. It has manufactures of calico.
Pop. 17,400.
EIMBECK ([=i]m'bek), or EINBECK, a town of Prussia, province of Hanover, 40
miles south of Hanover, once famous for its beer (_Eimbecker Bier_, whence
_Bock_). Pop. 9430.
EINSIEDELN ([=i]n'z[=e]-d[.e]ln), a village and district, Switzerland, in
the canton and 9 miles north by east of Schwyz, 3000 feet above the sea,
celebrated for its Benedictine abbey. An image of the Virgin, alleged to
possess miraculous powers, annually attracts immense numbers of pilgrims.
Pop. 8438.
EINSTEIN, Albert (1879- ), physicist, was born at Ulm, W[:u]rtemberg,
Germany, of German-Jewish parents. He was educated at the Gymnasium in
Munich, and, on leaving school in his sixteenth year, accompanied his
parents to Milan. Six months later, he enrolled at the Technical High
School in Zurich, where he studied from 1896 to 1900. He held a post in the
Swiss Patent Office from 1902 to 1909, then various professorships in
Zurich till 1914, when he received a call to the Prussian Academy of
Science, Berlin, as successor to van't Hoff. Einstein has been twice
married. He has become famous as the author of the Theory of Relativity
(q.v.). The 'special theory', which deals chiefly with electrodynamics and
optics, was published in 1905, and the 'general theory' or theory of
gravitation, about ten years later. His name became popularly known in
1919, after observations made during the solar eclipse of that year had
verified his prediction of the bending of rays of light coming from a star
and passing close to the sun. He has made other valuable contributions to
Theoretical Physics, among these being a theory of the Brownian movements
and various important applications of the modern quantum theory of energy.
In 1921 he visited the United States and Britain, and delivered many
lectures on Relativity.
EISENACH ([=i]'z[.e]n-[=a]_h_), a town of Germany, in the former
grand-duchy of Saxe-Weimar, near the mountains of Thuringia, at the
junction of the Nesse and H[:o]rsel. It is an attractive town, and contains
a palace erected in 1742. It has manufactures of pottery, leather, woollen
yarn, &c. Sebastian Bach was born there in 1685. Near it lies the Wartburg,
where Luther was kept for safety during 1521 and 1522. Pop. 38,362.
EISENBERG ([=i]'z[.e]n-ber_h_), a town of Germany, former duchy of
Saxe-Altenburg, with a palace and various manufactures. Pop. 10,750.
EISLEBEN ([=i]s'l[=a]-b[.e]n), a town, Prussian Saxony, 25 miles north-west
of Merseburg, celebrated as the place where Luther was born and where he
died. There are many memorials of Luther, and also a bronze statue of the
reformer erected in 1883. Copper is extensively worked in the
neighbourhood. Pop. 24,630.
EISNER, Kurt, Bavarian revolutionary leader, born in 1867 at Berlin, of a
Jewish family. He studied at the University of Marburg, and early acquired
a vast erudition. Entering journalism, he contributed to the _Frankfurter
Zeitung_, where he published an article attacking the Kaiser. For this he
was condemned to nine months' imprisonment. He then wrote for the Socialist
press and eventually became editor-in-chief of the _Vorw[:a]rts_. In 1907
he published a work entitled _The Fall of the Empire_ which attracted much
attention, and in 1910 he attacked Prussian ascendancy in the _Munich
Post_. Eisner took an active part in the Revolution of 1918, and was
appointed Prime Minister of Bavaria. A revolutionary and a Socialist, he
was, however, opposed to Bolshevism, which he did not hesitate to criticize
violently. But his policy of separation, his aim of liberating the South
German States, and his constant attacks upon the Kaiser and the whole
military caste of Germany, brought him many enemies. Whilst on his way from
his house to the Foreign Office he was shot at and killed by Count Arco
Valley on 21st Feb., 1919.
EISTEDDFOD ([=i]-steth'v[=o]d; W. _eistedd_, to sit, and _bod_, to be; pl.
_eisteddfodau_), an ancient assembly of Welsh bards for the purpose of
musical and poetical contests, the judges being originally appointed by
commissions from the native princes, and after the conquest from the
English kings. There are two kinds of eisteddfodau, the national or
general, and the provincial gatherings which take place in many parts of
Wales. The last commission was issued by Queen Elizabeth in 1568, but the
eisteddfod fell into abeyance during the seventeenth century. In 1798 the
ancient custom was revived by the Gwynnedigion Society, and on a more
elaborate scale by the Cambrian Society, which grew out of the
Gwynnedigion. Eisteddfodau are now held annually in North and South Wales
alternately, and are attended by many thousands of people. The festivals of
1919 and 1920 were held at Corwen and at Barry, Glamorgan,
respectively.--Cf. Rhys and Brynmor Jones, _The Welsh People_.
EJECT'MENT, in law, an action wherein the title to lands and tenements may
be tried and the possession recovered. It is commenced by a writ addressed
to the tenant in possession and all entitled to defend the possession,
bearing that the plaintiff lays claim to the property in question, and
calling upon all interested to appear within a certain time to defend their
rights. In its older form the action was remarkable for the curious
fictions on which procedure was based. The names of John Doe, an imaginary
plaintiff, and of Richard Roe, an imaginary defendant, were familiar in
cases of this kind in the English courts until 1852, when the Common Law
Procedure Act abolished these fictitious suitors.
EJECTOR, in mechanical engineering, an appliance for ejecting gases,
vapours, or liquids from closed spaces by the use of another gas, vapour,
or liquid at a higher pressure. For instance, the air may be extracted from
a condenser by an ejector. A jet of steam is directed along a short
specially-shaped pipe leading from the condenser to the outside atmosphere.
The velocity of the steam when it leaves the jet in the pipe is very high,
and it blows out into the atmosphere in spite of the atmospheric pressure
against it. In blowing out into the atmosphere it sucks the air in the
condenser along with it, and after it has been in operation some time
practically the whole of the air is sucked out of the condenser. The
appliance works on the principle of momentum. The active jet mixes with the
material to be ejected and imparts a common momentum to the mixture, which
is sufficient to enable it to pass outside the vessel from which the
ejection is taking place. The same principle is used in the mercury
air-pump (see _Air-pump_).--BIBLIOGRAPHY: W. E. Dalby, _Steam Power_;
_Modern Mechanical Engineering_ (The Gresham Publishing Company).
EKAT'ERINBURG, a town, Russia, in the government and 170 miles S.E. of
Perm, founded in 1723 by Peter the Great. It is the mining and metallurgy
centre of the Ural regions; and gem-cutting, the making of machinery,
cloth, and candles are industries. Pop. 70,000.
EKAT'ERINODAR, a town of Russia in the Caucasus, chief town of the Kuban
territory, on the River Kuban, a poorly-built place with a considerable
trade. Pop. 107,360.
EKAT'ERINOSLAV, a town of the Ukraine, capital of a government of the same
name, on the right bank of the Dnieper, 250 miles N.E. of Odessa. Founded
in 1787, it is a manufacturing centre, producing iron, machinery, tobacco,
and beer. Pop. 220,100.--The government, which is intersected by the
Dnieper and reaches the Sea of Azov, has great mineral wealth, especially
in coal (Donetz basin), iron, manganese, and rock-salt; and its fertile
black-earth soil produces abundant crops of wheat and other grains. Area,
24,478 sq. miles; pop. 3,537,300.
ELAEAGNA'CEAE, the oleaster family of plants, a small nat. ord. of
apetalous dicotyledons scattered over the northern regions. The only
British member is the sea-buckthorn (_Hippophae rhamnoides_).
ELA'IN, the oily principle of fat obtained by submitting fat to the action
of boiling alcohol, allowing the stearin to crystallize, and then
evaporating the alcoholic solution. It is not unlike vegetable oil in its
appearance and its properties, and forms soaps with alkalies.
E'LAM, the ancient name of a country on the eastern border of Babylonia.
Its civilization dates back beyond 3000 B.C. Before 2000 B.C. it was strong
enough to subdue part of Babylonia. Its power was finally broken by the
last Assyrian monarchs. The capital was Susa, which became prominent again
after the rise of Cyrus. Its splendour during the Persian period is
reflected in the _Book of Esther_, in which it is referred to as 'Shushan'.
[Illustration: Eland (_Oreas canna_)]
E'LAND, _Oreas_ (_Orias_) _canna_, a species of antelope inhabiting Africa,
the largest of all its kind, being about the size of an ox. Its flesh,
especially that of the thighs, which are dried and used in this state, is
highly prized. It is now almost extinct south of the Limpopo, but it is
plentiful in the Kalahari. The colour is a light or greyish brown, and it
possesses a short mane. The horns, which are about 18 inches long and
nearly straight, are spirally keeled.
EL'ANUS, the name of certain species of raptorial birds of the genus
El[=a]nus, belonging to the kites. The type species is the black-winged
kite (_E. caeruleus_) of Africa and South Asia, which strays to South-West
Europe. A very similar form (_E. leucurus_) is native to tropical and
subtropical America, and other species (_E. scriptus_ and _E. axillaris_)
are Australian.
ELAPHOMYCES, a genus of Ascomycetous fungi, section Plectascineae, with
closed, subterranean ascus-fruits resembling those of the genuine truffles
(Tuberineae), but not closely allied to that family. _E. cervinus_ (Hart's
truffle) is not infrequent in Britain; it forms 'mycorhiza' with roots of
oak, beech, and various conifers.
E'LAPS, a genus of poisonous American snakes, the type of the family
Elapidae, to which belongs the cobra de capello.
EL-ARISH, Egyptian city on the Mediterranean, on the Wadi el-Arish, and
chief city of the territory bearing the same name. It was taken by the
French under Kl['e]ber in 1799, but abandoned the same year. Pop. about
5000.
ELASMOBRANCHII (-brang'ki-i), a sub-class of fishes, including sharks,
dog-fishes, rays (skates); and also Chimaera (q.v.) and its allies. They
are predaceous forms, in which the mouth is a transverse slit on the under
side of the head, the numerous simple teeth are in several rows (except in
chimaeroids), the short intestine possesses a spiral valve and opens into a
cloaca. The tail is asymmetrical (heterocercal), and numerous placoid
scales (dermal denticles) are embedded in the skin. The skeleton is
cartilaginous; the heart possesses a muscular conus arteriosus with
numerous rows of pocket-valves; and there are five (sometimes six or seven)
pairs of gill-pouches opening by slits to the exterior, these not being
covered by an external flap (operculum) except in chimaeroids.
Fertilization is internal, and the male is provided with a pair of
copulatory organs (claspers) projecting backwards from the pelvic fins. The
eggs sometimes develop within the body of the mother, but are usually laid
in horny pouches (mermaids' purses). The group is of great antiquity, and
many extinct fossil types are known.
ELASMOTHE'RIUM, an extinct genus of Mammalia, found in the post-Pliocene
strata of Europe, comprising animals of great size allied to the
rhinoceros, and having probably one large horn and a smaller nasal horn.
ELASTIC BITUMEN, ELATERITE, or MINERAL CAOUTCHOUC, an elastic mineral
bitumen of a blackish-brown colour, and subtranslucent. It has been found
at Castleton, in Derbyshire.
ELASTICITY, the property in virtue of which bodies resist change of volume
or of shape, and tend to regain their original bulk or shape when the
deforming forces are removed. Solids possess elasticity of volume and of
shape. Liquids and gases have elasticity of volume; they resist
compression, but offer only a transient resistance to change of shape (see
_Viscosity_). The elasticity of a gas is measured by the pressure to which
the gas is subjected, if there is no change of temperature. When a gas is
compressed suddenly, it has a greater elasticity on account of the rise of
temperature which takes place. Liquids are less compressible than gases;
water is compressed by about 1 part in 20,000 when the pressure on it is
increased by one atmosphere. A knowledge of the elastic properties of
solids is of importance in all branches of applied mechanics. Homogeneous
solids offer definite resistance to compression, twisting, stretching, and
bending, and this resistance is expressed by a number called a modulus. Let
the deforming force be reckoned per unit of area, e.g. a pressure in tons
per square inch; this is called the stress. The unital deformation produced
by the stress is called the strain, for example, compression per unit of
volume. The modulus is obtained by dividing the stress by the strain; if
this is done with the above example, the ratio will give the bulk modulus.
When the applied forces cause change of shape without change of volume, the
ratio of stress to strain is called the shape modulus or the rigidity of
the material. This property is brought into play when mechanical power is
transmitted by means of shafting. Young's modulus is employed in the cases
of stretching and bending. It is given by the ratio stretching force per
unit area to stretch per unit length. In 1678 Hooke stated the law that
stress is proportional to the strain which it causes. This law is found in
practice to be true for metals within a certain range of stress which lies
below the elastic limit. If the stress is increased beyond this limit, the
material begins to give way, and permanent change of shape or volume takes
place. In the processes of riveting and wire-drawing, the material is
purposely strained beyond the elastic limit, whilst the correct working of
a spring balance requires that the spring should never be overstrained.
When metals are subjected to frequently repeated stresses, they undergo a
weakening and are said to become fatigued, and are liable to give way under
a smaller stress than would otherwise cause fracture. The speed with which
sound waves are transmitted through a material depends on the elasticity of
the material; such compressional waves in water have been employed by the
Roumanian engineer, Constantinescu, to transmit power by means of
water-pipes.
_Mathematical Theory._--Consider an elastic body at rest and free from
strain. Let the body be subjected to forces, fulfilling the ordinary
statical conditions of equilibrium, and therefore not tending to give the
body any motion of translation or rotation as a whole. The particles of the
body will move very slightly relative to each other; in other words, a
system of strain will be set up in the body. To maintain this strain a
definite system of stress is necessary. The problem for the mathematical
theory is to determine the state of strain and stress at every point of the
body when the applied forces are given. These applied forces may either be
body forces (of which practically the only example is weight), or surface
forces; the latter are pressures or tractions, and are defined by their
directions and amounts per unit area. It is first of all necessary to show
how strain and stress can be specified mathematically.
_Strains._--If x, y, z are the co-ordinates of a point in the unstrained
body, and if this point is displaced to (x + u, y + v, z + w) when the
straining forces are applied, then u, v, w, which are supposed to be very
small, are called the component displacements at (x, y, z). It is clear
that if u, v, w were constant, the body would simply be displaced without
strain. The state of strain can in fact be shown to depend on the first
derivatives of u, v, w with respect to x, y, z. The strain round any given
point consists simply of three stretches parallel to a certain set of three
mutually perpendicular directions. These directions vary from point to
point, so that this specification of the strain is inconvenient for
calculations. A suitable method depends on the fact that the strain is
known round a point when we know the values of the six quantities
du/dx, dv/dy, dw/dz, dw/dy + dv/dz, du/dz + dw/dx, dv/dx + du/dy,
at the point. These are called the components of strain at (x, y, z). The
first three are _stretches_ parallel to the axes, the other three are
_shearing_ strains. We may get an idea of the nature of these strains from
two simple typical cases. 1. Let u = ex, v = 0, w = 0. This makes du/dx =
e, and the other five strains zero. But we see that the displacement of
every particle is perpendicular to the yz plane, and proportional to its
distance from that plane. Every line parallel to the axis of x is therefore
elongated by a definite fraction of its original length, the value of this
fraction being e, which is du/dx. The strains du/dx, dv/dy, dw/dz are
therefore _stretches_ parallel to the axes. 2. Let u = 0, v = cz, w = 0.
This gives dv/dz + du/dy = c, and the other five strains zero. The
displacement of every particle is parallel to the axis of y, and
proportional to its distance from the plane xy. The strain is therefore a
slide, or _shear_ of planes parallel to xy in the direction of the axis of
y.
_Stresses._--To specify the stress round a point (x, y, z), consider a
small plane area through the point. The material on one side of this acts
on the material on the other side with a certain force whose components
parallel to the axes are F, G, H, say, per unit area. If we know F, G, H
for every orientation of the small plane area, the state of stress round
(x, y, z) is defined. But it is easy to show, as below, that we can find F,
G, H for every area if we know them for areas parallel to the co-ordinate
planes. We are thus led to the specification of the stress round (x, y, z)
by the six _components of stress_, [=xx], [=yy], [=zz], [=xy], [=xz],
[=yz]; where [=xy], for example, means the force per unit area _parallel_
to Ox exerted on a plane _perpendicular_ to Oy by the material on the
positive side of that plane on the material on its negative side. Thus e.g.
the components of the force per unit area exerted across the yz plane
through (x, y, z) by the material on the positive side of that plane are
[=xx], [=xy], [=xz]. It is important to note that [=xy] = [=yx]. This is
easily proved by considering the equilibrium of a small rectangular volume
of the material round (x, y, z) as centre, with its edges parallel to the
axes; if [=xy] were not equal to [=yx], there would be a residual couple in
the plane xy.
_Relations between the Strains and the Stresses._--When the strains are
known, the stresses can be found from a generalized Hooke's law, which can
be deduced from the principle of energy, combined with consideration of
symmetry. If the solid is isotropic, i.e. is symmetrical in its elastic
properties in all directions round a point, the relations between stress
and strain are of the form
[=xx] = [lambda](du/dx + dv/dy + dw/dz) + 2[mu]du/dx,
[=yz] = [mu](dw/dy + dv/dz);
the values of the other four components of strain can be written down from
symmetry. Here [lambda] and [mu] are constants, each being a _modulus_ of
elasticity. In particular [mu] is the _shape modulus_ or the _rigidity_,
already referred to. The Young's modulus and the bulk modulus can easily be
found in terms of [lambda] and [mu].
_Equations of Equilibrium in Terms of the Stresses._--A rectangular element
dx, dy, dz, of the body is held in equilibrium by the body force, and the
tractions on its faces arising from the stress. The tractions per unit
area, parallel to the axis of x, on the six faces, are: on the plane x,
-[=xx]; on the plane x + dx, [=xx] + (d/dx [=xx])dx; on the plane y,
-[=xy]; on the plane y + dy, [=xy] + (d/dy [=xy])dy; and similarly for the
plane z. Let the force acting on the mass of the body, such as its weight,
be (X, Y, Z) per unit mass, and let [rho] be the density. By equating the
sum of the x components of all the forces to zero, we get
d[=xx]/dx + d[=xy]/dy + d[=xz]/dz + [rho]X = 0;
Similarly d[=xy]/dx + d[=yy]/dy + d[=yz]/dz + [rho]Y = 0,
and d[=xz]/dx + d[=yz]/dy + d[=zz]/dz + [rho]Z = 0.
_The Surface Tractions in Terms of the Stresses._--Draw a small tetrahedron
round (x, y, z) with its faces perpendicular to the axes and to the
direction (l, m, n), and consider the equilibrium of this small body. Let
F, G, H be the components per unit area of the force on the plane whose
direction cosines, _drawn outwards_, are l, m, n. If A be the area of the
face perpendicular to (l, m, n), we get, by resolving parallel to Ox,
F.A = [=xx].lA + [=xy].mA + [=xz].nA.
Hence F = l[=xx] + m[=xy] + n[=xz],
and similarly G = l[=xy] + m[=yy] + n[=yz],
H = l[=xz] + m[=yz] + n[=zz].
If (x, y, z) is a point at the surface of the body, and l, m, n are the
direction cosines of the outward normal at that point, these values of F,
G, H are the component of the force that must be applied from outside to
the surface at (x, y, z) to maintain the state of stress.
_The Equations connecting the Displacements and the Applied Forces._--By
using, in the equations of equilibrium, the values of the stresses in terms
of the strains, we find the body equations of equilibrium in terms of
displacements,
[mu](d^2u/dx^2 + d^2u/dy^2 + d^2u/dz^2) + ([lambda] + [mu])d/dx(du/dx +
dv/dy + dw/dz)
+ X[rho] = 0,
with two similar equations.
The surface equations of equilibrium can also be written down at once by
substituting the values of stresses in terms of strains in the expressions
for F, G, H given above.
_The Problem of Equilibrium._--To find the strain under given forces we
have to solve the body and surface equations of equilibrium, when X, Y, Z
and F, G, H are given. This problem has not been completely solved except
in a few cases. It was solved by Lam['e] and Lord Kelvin for a solid or
hollow sphere; it has also been solved for an infinite solid bounded by two
parallel planes, or by a circular cylinder. Many particular solutions,
however, are known for bodies of other shapes. Some of these solutions are
of great practical value, e.g. St. Venant's solutions for the torsion and
flexure of prisms. For bodies in which one or two dimensions are small,
i.e. for thin plates and shells, and for thin rods, approximate theories
have been given, which are partly deduced from the above exact equations,
and partly from plausible hypotheses, a complete treatment based on the
exact equations being in most cases impracticable.
_The Problem of Vibrations._--When a body is vibrating, the mass
acceleration parallel to Ox of the particle at (x, y, z) is [rho] dx dy dz
d^2u/dt^2. The equations of vibration are therefore found by writing -
d^2u/dt^2 instead of X, in the first body equation of equilibrium, and
similarly with the others. The surface conditions will usually be that F,
G, H are zero. The problem has been completely solved by H. Lamb for a
solid or hollow sphere. For the elastic solid theory of the luminiferous
ether, see _Ether_; for some practical solutions of the general equations
of equilibrium, see _Strength of Materials_.--BIBLIOGRAPHY: A. E. H. Love,
_Mathematical Theory of Elasticity_; Lord Kelvin and Tait, _Natural
Philosophy_; Todhunter and Pearson, _History of Elasticity and Strength of
Materials_.
EL'ATERIDAE, the name of a family of beetles, remarkable for their ability
to throw themselves to a considerable height in the air, when placed on
their back, by a vigorous muscular movement. Hence their names of
springing-beetles, click-beetles, skip-jacks, &c. When alarmed, the elater
counterfeits death. Flowers, grass, and decaying wood are the habitations
of these animals, which are almost always found singly. The larvae are
often very injurious to vegetation, especially those which devour the roots
of herbaceous plants (as in the genus Agri[=o]tes), and are known from then
slenderness and hardness as wire-worms. The fireflies of America belong to
the family. In these a pair of luminous organs is found on the thorax,
while there is a third on the under side of the base of the abdomen. The
_Pyroph[)o]rus noctil[=u]cus_, called _cucujos_ in Brazil, is used as a
personal ornament by ladies. The largest species of the genus Elater, the
_Elater flabellicornis_, is 2-1/2 inches in length.
ELATE'RIUM, a substance obtained from the fruit of the squirting or wild
cucumber (_Ecballium agreste_). The juice of the unripe fruit, when
expressed and allowed to stand, deposits elaterium as a green sediment with
an acrid taste, a faint odour, and powerful cathartic properties. It is a
violent purgative, and is poisonous, but its action is not constant. The
active principle in it is called _elaterin_.
ELBA (Lat. _Ilva_), a small island in the Mediterranean, in the province of
Livorno (Leghorn), Italy, separated from the mainland by the Strait of
Piombino, about 6 miles wide. The island is 18 miles long and from 2-1/2 to
10-1/2 miles broad, and is traversed by mountains rising to a height of
over 3000 feet. It is rich in iron, marble, granite, salt, &c.; and iron
ore is exported. Excellent wine and fruits are produced. It has two
seaports--Porto-Ferrajo (the capital) and Porto-Longone. The Treaty of
Paris in 1814 erected Elba into a sovereignty for Napoleon, who resided in
it from 4th May, 1814, to 26th Feb., 1815, when he escaped and landed at
Cannes on 1st March. After Napoleon's departure the island was restored to
Tuscany, which became part of Italy in 1860. Pop. 30,450.
ELBE (elb; Ger., pronounced el'be; Lat. _Albis_; Bohem. _Labe_), an
important river in Central Europe. It rises on the south-west slopes of the
Schneekoppe or Snowcap, one of the Riesengebirge, between Bohemia and
Silesia. From this point it flows nearly due south into Bohemia for about
50 miles, when it turns to the west, and after about 40 miles takes a
general north-north-west direction till it falls into the North Sea,
intersecting Saxony and a considerable portion of Prussia. The finest
scenery of its valley is in the Saxon Switzerland. Its length is 725 miles;
drainage area, 56,865 sq. miles. The principal affluents are: on the right,
the Iser, Schwarz-Elster, and Havel; on the left, the Alder, Moldau, Eger,
Mulde, and Saale. In the lower part of its course the river divides into
several arms, which unite again about 5 miles below Hamburg. It is more or
less navigable for about 525 miles, but its estuary is much encumbered with
sand-banks. In 1870 its navigation was declared free from Hamburg to Melnik
in Bohemia. The North Sea and Baltic ship canal connects its estuary with
Kiel Bay, and there are other important connected canals. It is well
stocked with fish.
ELBERFELD (el'b[.e]r-felt), a town of Rhenish Prussia, in the government of
and 15 miles east of D[:u]sseldorf, on both sides of the Wupper, enclosed
by lofty hills. Taken with Barmen it stretches along the Wupper valley for
about 7 miles. The old streets are narrow and irregular, but the newer
quarters are well built. It is a great seat of manufacturing industry,
among its leading products being cottons, woollens, silks, velvet, mixed
textile goods, buttons, ribbons, lace, yarns, thread, carpets, aniline
dyes, iron and steel, machinery, pianofortes, and paper. Calico-printing,
dyeing, and bleaching are very extensively carried on. It has given its
name to a system of poor relief, combining organized voluntary effort and
individual treatment. Pop. 170,195.
ELBEUF (el-beuf), a town of France, department of Seine-Inf['e]rieure, 11
miles S.S.W. of Rouen, in a valley on the left bank of the Seine, connected
by two bridges with St. Aubin on the opposite side of the river. It is an
important centre for the production of woollen manufactures, chiefly of
lighter cloths and fancy goods, and is also an entrep[^o]t for the finer
and heavier cloths of Louviers and Sedan. It communicates by steamers with
Paris, Rouen, and Havre. Pop. 19,240.
ELBING, a seaport town of West Prussia, on the Elbing, near its entrance
into the Frische-Haff. It was once a flourishing Hanse town, and is still a
place of considerable industry and trade, the manufactures including iron
goods, machinery, brass and tinplate goods. It has also shipbuilding yards.
Pop. 58,636.
ELBURZ, a lofty mountain range extending over Northern Persia, parallel
with and overlooking the Caspian. Highest peak, Mt. Demavend, 19,400 feet;
average height, 6000 to 8000 feet.
ELCESAITES (el-ses'a-[=i]ts), a sect of Gnostics which arose in the reign
of Trajan about the beginning of the second century. They were a branch of
the Essenes, and resembled the Ebionites. A Jew, named Elxai, or Elkesai,
is their reputed founder.
ELCHE (el'ch[=a]), a town of Spain, in the province and 14 miles W.S.W. of
Alicante, on the left bank of the Vinalopo, surrounded by palm trees. It
contains various Roman remains, a fine church, and a town house of the
fifteenth century. Chief industry, the culture of dates. Each summer, from
the 13th to the 15th of August, an interesting f[^e]te is held at Elche,
and a fourteenth century liturgical drama (_The Representation of the
Assumption of Our Lady St. Mary_) is performed. Pop. 27,620.
ELCHINGEN (el'_h_ing-en), OBER and UNTER, two villages of Bavaria, on the
left bank of the Danube, about 3 miles apart and 8 miles north-east of Ulm.
In 1805 Marshal Ney defeated the Austrians at Ober Elchingen, and won for
himself the title of Duke of Elchingen.
[Illustration: Common Elder: Foliage, Inflorescence, and Fruit]
ELDER, a name given to different species of the genus Samb[=u]cus, nat.
ord. Caprifoliaceae. These are small trees or shrubs, with opposite and
pinnated leaves, bearing small white flowers in large and conspicuous
corymbs, small berries of a black or red colour, and bitter and nauseous
leaves possessing purgative and emetic properties. The wood of the young
shoots contains a very large proportion of pith. The common elder of
Britain (_S. nigra_) is a wild shrub or small tree, distinguishable by its
winged leaves, its clusters of small, cream-white flowers, and the small
black berries by which these are succeeded, and from which a kind of wine
is sometimes made. The dwarf elder or danewort (_S. Ebulus_) is also found
in many parts of Britain, and was popularly supposed to have sprung from
the blood of the Danes. Two species inhabit North America: _S. canadensis_,
a common plant from the 49th to the 30th parallel of latitude, the berries
of which are black and have a sweet taste; and _S. pubescens_, which bears
red berries, and inhabits Canada, the northern parts of New England, and
the Alleghany Mountains. Elder wood is yellow, and in old trees becomes so
hard that it is often substituted for box-wood. Its toughness, also, is
such that it is made into skewers and tops for fishing-rods. The light pith
is utilized for balls for electric experiments, and various ointments,
drinks, and medicinal decoctions are made from the bark, leaves, flowers,
and berries.
ELDERS, persons who, on account of their age, experience, and wisdom, are
selected for office, as, among the Jews, the seventy men associated with
Moses in the government of the people. In the modern Presbyterian Churches
elders are officers who, with the pastors or ministers, compose the
consistories or kirk-sessions, with authority to inspect and regulate
matters of religion and discipline in the congregation. As a member of the
kirk-session the elder has an equal vote with his minister, and as a member
of the higher Church courts, when delegated thereto, he has a right to
discuss and vote on all matters under discussion in the same manner as the
clergy themselves. In the Mormon Church the elder is an officer whose duty
it is "to preach and baptize, to ordain other elders, to bless children,
and to take the lead at all meetings". Among the Shakers there are four
elders, two men and two women, in each congregation.
ELDON, John Scott, Earl of, Lord Chancellor of England, born in 1751 at
Newcastle-on-Tyne, died in London, 13th Jan., 1838. His father was a
coal-dealer and public-house keeper of means, and John was educated with
his brother William (afterwards Lord Stowell) at Newcastle, and at Oxford,
where he obtained a fellowship. He was called to the Bar in 1776, and in
1782 was made King's Counsel. Next year he entered Parliament, supported
Pitt, and was made Solicitor-General, and knighted. In 1792 he purchased
the estate of Eldon. In 1793 he became Attorney-General, and in 1799 was
created Chief Justice of the Court of Common Pleas, and raised to the
peerage and the House of Lords by the title of Baron Eldon. On the
accession of the Addington ministry he became Lord Chancellor (1801), and
retained this post under the subsequent administration of Pitt until the
death of the latter in 1806. A year later, however, he resumed the
chancellorship under Liverpool, and held it without break for twenty years.
In 1821 he was created an earl by George IV. On the accession of the
Canning ministry in 1827 he resigned the chancellorship, and never again
held office. As a lawyer he was a master of English jurisprudence; as a
politician he was opposed to reform, and by no means free from the charge
of servility and intrigue.
EL DORA'DO, a country that Orellana, the lieutenant of Pizarro, pretended
he had discovered in South America, between the Orinoco and Amazon Rivers;
and which he named thus on account of the immense quantities of gold and
precious metals which, he asserted, he had seen in Manoa, the capital of
the country. The term El Dorado was first applied to a South American
tribal king who was said to cover his body annually with gold-dust. It now
designates any place abounding in gold, or offering opportunities for the
acquisition of sudden wealth.
ELEANOR CROSSES (el'i-nor), memorial crosses erected on the spots where the
bier of Eleanor, the wife of Edward I, rested on its way from Grantham to
Westminster. Twelve were erected, but only three, those of Northampton,
Geddington, and Waltham, remain.
ELEAT'IC SCHOOL, a Grecian philosophical sect, so called because it
originated in Elea (Lat. _Velia_), a town of Magna Graecia (Southern
Italy), of which also three of its most celebrated teachers, Parmenides,
Zeno, and Leucippus, were natives. The founder was Xenophanes of Colophon,
who came to Elea late in life, bringing with him the physical theories of
the Ionian school, to which he added a metaphysic. The two schools soon
drifted widely apart, especially in respect of method. In opposition to the
physical philosophy of the Ionian school, and also the doctrine of
Heraclitus, who taught that everything is flux, the Eleatic philosophers
asserted that change and difference are only empty illusions, and that the
only true reality is changeless being. Starting from the observation of
external nature, the Ionians endeavoured to discover some elementary
principle, as water, air, fire, or a combination of elements, by the action
of which the phenomena they observed might be accounted for. The Eleans
made the abstract idea of Being or God, deduced from the contemplation of
the Universe as a whole, their starting-point. Their reasonings sometimes
led them to deny the reality of external phenomena altogether.
ELECAMPANE (el-i-kam-p[=a]n'; _In[)u]la Hel[=e]nium_), a plant of the nat.
ord. Compositae, found in Britain and other parts of Europe, and in Asia.
It is 3 or 4 feet high; the radical leaves are often 2 feet and more in
length; the flowers are large and yellow; the root, which is perennial,
possesses a bitter camphor-like taste. It was formerly much used as a
stimulant for all the secreting organs, and in tuberculosis on account of
the germicidal action of the bitter principle (helenin) which it contains.
ELECTION, in theology, the doctrine that God has from the beginning elected
a portion of mankind to eternal life, passing by the remainder. It is
founded on the literal sense of certain passages of Scripture, and has been
amplified by the labours of systematic theologians into a complete and
logical system. It dates in ecclesiastical history from the time of
Augustine; but Calvin has stated it so strongly and clearly in his
_Institutes_ that it is generally associated with his name.
ELECTION, in politics, the selection by voting of a person or persons to
occupy some post or office. The most important elections are those of the
members of the legislative assemblies of the different countries, and as to
the manner in which these are carried out strict laws are in force. In such
elections voting by ballot is now general. The chief forms of election in
Britain are parliamentary and municipal elections, in both of which the
basis of the suffrage (or right of voting) is the payment of poor rates.
Members of Parliament formerly required a property qualification in England
and Ireland; but this restriction, which never existed in Scotland, has
been abolished. In both parliamentary and municipal elections the ballot
has been in operation since 1872. For the prevention of bribery and corrupt
practices many Acts have been passed, of which that now in operation came
into force in Oct., 1883, and has been annually renewed. By it persons
convicted of treating, bribery, personation, and undue influence are liable
to imprisonment with hard labour, and to disqualification in respect of the
franchise and public offices. It also imposes many limitations with regard
to the number of assistants and committee-rooms, and the use of
conveyances. By the Reform Act of 1918, the maximum expenditure for
campaign purposes during parliamentary elections is to be sevenpence per
elector in county constituencies, and fivepence per elector in boroughs. By
this Act the cost of registration is paid half out of local rates, and half
by the State. At election times the returning officer's expenses are to be
paid by the Treasury. Under the provisions of the Ballot Act the returning
officer is required, in the case of a county election, to give notice of an
election within two days after that on which he receives the writ; or in
the case of a borough election, to give notice on the day on which he
receives the writ, or at the latest on the day following. In county or
district borough elections the nomination must take place within ten days
of the receipt of the writ, at least three clear days, however, being
allowed to elapse between the first public notice and the day of
nomination. In ordinary borough elections the candidate must be nominated
not earlier than the third day after public notice, and not later than the
fourth day after that on which the writ is received. A candidate is
nominated in writing, with the signatures of a proposer, seconder, and
eight other electors, all registered in the constituency to be represented.
In the event of there being more candidates than vacancies, the returning
officer adjourns the election for the purpose of taking a poll. The polling
must take place not less than two or more than six clear days after the day
of nomination, if it be a county or district borough election; in the case
of an ordinary borough, it must take place not more than three clear days
after nomination, Sundays, Christmas Day, &c., not being counted as days.
Where the votes for rival candidates are equal, the returning officer, if
registered in the constituency, may give the casting-vote. If he decline to
do so, a scrutiny is demanded, which usually results in certain deductions
on the ground of spoiled papers, disqualified voters, &c., sufficient to
give one candidate priority. In elections for the school boards the
cumulative system of voting is employed (see _Cumulative Vote_).--Cf. C.
Seymour and D. P. Frary, _How the World Votes_.
ELECTOR (Ger. _Kurf[:u]rst_, 'electoral prince'), the title of certain
princes of the Holy Roman Empire, who had the right of electing the
emperors. In the reign of Conrad I, King of Germany (912-918), the dukes
and counts became gradually independent of the sovereign, and assumed the
right of choosing future monarchs. In the thirteenth century the number of
these electors was seven--the Archbishops of Mainz, Cologne, and Tr[`e]ves,
the King of Bohemia, the Count Palatine, the Duke of Saxony, and the
Margrave of Brandenburg. In 1648 an eighth electorate was created to make
room for Bavaria, and Hanover was added as a ninth in 1692. The votes of
the Palatinate and of Bavaria were merged in one in 1777. In 1802 the two
ecclesiastical electors of Cologne and Tr[`e]ves were set aside, and Baden,
W[:u]rtemberg, Hesse-Cassel, and Salzburg declared electorates; so that
there were ten electors in 1806 when the old German Empire was
dissolved.--Cf. Viscount Bryce, _The Holy Roman Empire_.
ELECTRICAL FISHES, a name given to fishes possessing the property of
communicating an electric shock when touched with the hand or any electric
conductor. One of the best known is the electric eel (_Gymn[=o]tus
electricus_), a native of South America. It is of nearly equal thickness
throughout; head and tail obtuse; ordinary length, 3-1/2 to 4 feet. The
seat of the four electrical organs is along the under side of the tail, and
they are said to possess the power of knocking down a man, and of painfully
numbing the affected limb for several hours after the shock. After a few
discharges, however, the faculty of producing a shock is impaired, and an
interval of rest is required for a new storage of force. Similar but
less-marked powers are possessed by an African cat-fish (_Malapterurus
electricus_), in which the electric organ invests the entire body as a sort
of jacket under the skin. Still feebler in this respect are the electric
rays, of which the best known (species of Torpedo) are native to the
Mediterranean, Red Sea, Atlantic, and Pacific Oceans. Here the electric
organ consists of a large mass on each side of the front part of the body.
ELECTRICAL MEASURING INSTRUMENTS, the name given to instruments which
measure electric power, energy, voltage, or current. The majority of such
instruments are current-operated. Thus, with the exception of electrostatic
voltmeters, all voltmeters are really current measuring instruments; but
since this current is made to be proportional to the P.D. between the
voltmeter terminals, the scale reading is proportional to the voltage being
measured. One and the same instrument may be used as an ammeter or as a
voltmeter, by providing it with shunts for use as an ammeter, and series
resistances for use as a voltmeter. If the current to be measured is large,
the _shunt_ will have a very _low_ resistance compared with that of the
instrument, so that only a small fraction of the total current passes
through the instrument. Similarly, when a large P.D. is being measured, the
_series resistance_ will have a very _high_ value, so that the current
through the instrument may not exceed that which gives full-scale reading.
By using shunts or series resistances of different values, different ranges
can be given to the instruments.
In addition to the types already described (see _Ammeter_), there is a
class depending on the mutual action of current-carrying conductors placed
near one another. This type is largely used in alternating-current work. It
is also specially suitable for power measurements, and practically all
_wattmeters_ work on this principle. The Siemens Dynamometer was the first
instrument of this type.
The Kelvin Standard Balance is a special form of dynamometer, in which the
mechanical turning-moment due to weights on a beam is balanced by the
electrical turning-moment due to currents in fixed coils and in coils
attached to the ends of the beam. The electrostatic voltmeter mentioned
above is the only instrument which is operated by a P.D. instead of a
current. In it a moving vane is attracted into a fixed pair of quadrants;
or a set of vanes is attracted into a set of quadrant cells.
The majority of electricity meters are of the motor type, i.e. a disc is
driven by motor action at a speed which is proportional to the power
passing through the meter. The disc spindle engages with gearing which
drives the pointers on a set of dials recording the energy units. There are
also meters depending on electrolytic action (Wright meter); or on the
difference in period between two pendulums, one of which is controlled by
the load current (Aron meter).
ELECTRIC BATTERY, a group of primary or secondary cells, suitably arranged
for the purpose of producing an electric current. Primary batteries
consisting of a few cells are commonly used for intermittent work where a
relatively small current is required, e.g. for electric bells. If a larger
current is necessary, especially if it has to be maintained over a
considerable period, a battery of secondary cells is used. Such batteries
are commonly used for country house lighting. Very large batteries, used
either alone or in conjunction with automatic reversible boosters, are
frequently employed in public electric supply systems.
The name _electric battery_ was originally given to an arrangement of
Leyden jars (see _Leyden Jar_), but is now applied only to cells, the
Leyden-jar arrangement being called a Leyden-jar battery.
ELECTRICITY, the name given to the ultimate cause of electrical phenomena.
The laws governing these phenomena are well known, but the actual nature of
electricity has not yet been fully revealed, although much light has been
thrown on the subject by recent researches. (See _Electron_.) Although the
practical applications of electrical phenomena have all been developed
within the last fifty years, the production of an electric charge by
friction, as demonstrated by the power of rubbed amber to attract light
bodies, was observed by a Greek philosopher as long ago as 600 B.C. The
Greek name for amber, [Greek: elektron] (electron), is the root from which
our word electricity is derived. Friction was the only artificial source of
electricity known until Galvani, near the close of the eighteenth century,
accidently obtained it by the contact of two metals with the limbs of a
frog; and Volta, developing Galvani's discovery, invented the first
galvanic or voltaic battery.
The discovery by Faraday in 1831 of the principle of the production of an
electromotive force by the motion of a conductor in a magnetic field, laid
the foundation for the development of the electric generator (q.v.), and
thus of modern electric power supply.
The study of electrical phenomena is conveniently divided into two
branches, one dealing with stationary charges of electricity
(_electrostatics_), the other with electric currents (_current
electricity_).
_Electrostatics._--If a pair of ebonite rods be electrified by friction
with flannel, then by suspending the one rod and presenting the other to
it, it is easily demonstrated that a mutual mechanical force of repulsion
exists between them. If now a glass rod be electrified by friction with
silk, it will be found that it attracts the suspended electrified ebonite
rod. These experiments reveal the facts that electric charges may be of two
opposite kinds, and that like charges repel one another, while unlike
charges attract one another.
The charge produced on glass by friction with silk is called _positive_;
that produced on ebonite by friction with flannel is called _negative_. The
kind of charge produced depends not merely on the material rubbed, but also
on the material of the rubber. Thus a warm dry glass rod becomes
_negatively_ electrified when rubbed with fur. The rubber always becomes
electrified with a charge of the _opposite_ kind to that produced on the
material rubbed, and these two charges are _equal in amount_. All bodies
may be electrified by friction, but those which allow a free movement of
the charge over them (such bodies are called _conductors_, to distinguish
them from _insulators_, which do not allow this free movement) must be held
by an insulating handle, or else the charge will be removed as quickly as
it is produced.
Coulomb proved that the magnitude of the mutual mechanical force exerted
between two charged bodies depends on the amounts of the charges and the
distance between them. Faraday called attention to the influence of the
medium in which the charges are placed. Thus if two charges of q_1 and q_2
units respectively are placed d centimetres apart in a given medium, the
mechanical force f in dynes exerted between them is given by the equation f
= (q_1q_2)/(Kd^2), provided the dimensions of the bodies on which the
charges are concentrated are small in comparison with d. The coefficient K
is called the _dielectric constant_ of the medium, and its value is taken
as unity for air.
In accordance with this relationship, _unit charge_ is defined as that
charge which repels an equal and similar charge placed at a distance of 1
centimetre in air, with a force of 1 dyne.
If the medium surrounding a charged body be explored with a unit charge, a
mechanical force varying in magnitude and direction from point to point
will be found to act on the unit charge. In such a case, an _electric
field_ is said to exist in the medium.
The _strength of the electric field_ at any point is defined as numerically
equal to the mechanical force which would act on a unit charge placed in
air (or more strictly in a vacuum) at that point. The _direction_ of the
electric field at any point is defined to be the direction of the
mechanical force acting on a unit _positive_ charge placed at that point.
It should be noted that the strength of the electric field and the
mechanical force are numerically equal only when the dielectric constant of
the medium is unity. Thus if F is the field strength, K the dielectric
constant, and U the mechanical force acting on a unit charge, F = KU.
It is very convenient to represent an electric field by means of what are
called _lines of electric force_. If lines are drawn, starting from a
positive charge and ending on a negative charge, such that the tangent to
the line at any point is the direction of the electric force at that point,
these lines are called lines of electric force. They can be drawn in such a
way that the strength of the electric field at any point is numerically
equal to the number of lines of electric force passing through unit area
surrounding that point (and taken at right angles to the direction of the
force). The lines of electric force will thus completely represent the
electric field.
Further, if the following properties are attributed to the lines of
electric force, viz. (_a_) that a line of electric force tends to shorten
itself as far as possible; (_b_) that lines of electric force mutually
repel one another; then all the phenomena due to the presence of an
electric field may be interpreted by the behaviour of the lines of electric
force.
Figs. 1 and 2 show the lines of electric force in the space surrounding two
charged spheres. Fig. 1 shows the case where the charges are opposite, fig.
2 the case where they are similar. In fig. 1 the attraction between the
spheres may be thought of as due to the tendency of the lines of force to
shorten themselves. Similarly, the mutual repulsion of the spheres in fig.
2 may be regarded as a consequence of the mutual repulsion of the lines of
force.
[Illustration: Fig. 1]
[Illustration: Fig. 2]
The distribution of a charge upon an insulated conductor isolated in space
depends upon the shape of the conductor. If the conductor is spherical, the
charge is uniformly distributed. If the curvature varies from point to
point, the quantity of charge per unit area, or the _electric surface
density_, will vary from point to point. The sharper the curvature is, the
greater the surface density will be. In fig. 3 the distance of the dotted
lines from the surface of the conductors is proportional to the surface
density. These lines, therefore, give a graphical representation of the
distribution of charge. In sharply pointed conductors nearly the whole
charge will be concentrated at the pointed end. Owing to the large charge
per unit area at the pointed part, particles of dust, water-vapour, &c.,
will be powerfully attracted, will become charged by conduction, and will
then be powerfully repelled. In this way the original charge will be
rapidly dissipated. This effect may be shown by keeping a sharply pointed
conductor powerfully charged by an electric machine. The streaming of the
particles from the point produces a wind which is sufficient to blow out
the flame of a candle.
[Illustration: Fig. 3]
Conductors which are intended to retain their charge for a long period must
be smooth and polished, and the maximum curvature must be as small as
possible. In lightning-conductors practical advantage is taken of this
'power of points' to dissipate a charge rapidly.
The distribution of the charge on a conductor is influenced by the presence
of other conductors, whether charged or not. This is due to what is called
_electrostatic induction_. If an uncharged insulated conductor B is brought
near a charged conductor A, a charge of the _opposite_ kind is induced on
the parts of B nearer to A, and a charge of the _same_ kind on the parts
farther away from A. Since B was originally uncharged, these induced
charges are equal in amount.
If B is now removed to a distance, the induced charges neutralize one
another, and B returns to its original uncharged state.
While B is near A, let the induced charge of the _same_ kind as the charge
on A be neutralized by touching B with an earth-connected conductor, say
the finger. On removing B to a distance, it will no longer be uncharged as
before, but will have a charge of the _opposite_ kind from that on A. B is
now said to have been _charged by induction_.
It is instructive to view these phenomena in the light of the conception of
lines of electric force. When B is brought up towards A, some of the lines
of force associated with the charge on A, and originally linked to
surrounding objects, will now, owing to the tendency of the lines to
shorten themselves, be linked to B. At the same time an equal number of
lines (of opposite direction relative to B) will link B to the nearest
surrounding objects.
Since by definition a line of force starts from a positive charge and ends
on a negative charge, the charge on the parts of B nearer to A will be of
the _opposite_ kind to that on A, but the charge on the part farther from A
will be of the _same_ kind as that on A. When the earth-connected conductor
is brought near B, the lines formerly linking B to surrounding objects will
link B to the earth-connected conductor. Finally, when the latter touches
B, these lines shorten themselves indefinitely and disappear.
The attraction of light particles to a charged body is explained by
electrostatic induction. The charge of opposite kind induced on the
particle being nearer than that of the same kind, the particle is
attracted. When it touches the charged body, the charge of opposite kind is
neutralized, and the charge of like kind now left on the particle causes
repulsion to take place. If the electrified body is an insulator, the
neutralization of the charges only takes place slowly, and consequently it
may be some time before the particle is repelled.
[Illustration: Fig. 4.--Induction, and Lines (or tubes) of Force]
If two charged conductors be connected by a wire, in general it will be
found that a flow of electricity from one to the other will take place.
This flow is said to be due to a _difference of electric potential_ between
the two conductors. If no flow takes place, then the difference of
potential is zero. Electric potential difference (the contraction P.D. is
commonly used) is numerically equal to the work done in carrying a unit
charge from the one conductor to the other. If the work is done _against_
the electric forces, in moving a unit positive charge from A to B, then B
is said to be at a higher potential than A. Although actually it is with
_differences_ of potential that we have always to deal, it is convenient in
many cases to refer these differences to a zero, and speak of _the
potential_ at a point. The ideal zero of potential would be the potential
at a point infinitely far removed from all electrified bodies. In practice
it is convenient to regard the potential of the earth as zero. The
potential at a point is then numerically equal to the work done in carrying
a unit positive charge from earth to the point. The potential at every
point on a conductor is obviously the same, for if it were not so, a flow
of charge would take place and equalize the potential. If an insulated
uncharged conductor be connected by a wire to a charged conductor, a flow
of charge will take place until every point on both conductors is at the
same potential. The quantity of charge which each conductor will then have
depends on what is called the _capacity_ of the conductor.
The _capacity of a conductor_ is defined as the quantity of electricity
with which it must be charged in order to raise its potential from zero to
unity. Thus if Q be the quantity, V the potential, and C the capacity, we
have C = Q/V. The potential of a conductor is, therefore, directly
proportional to the charge upon the conductor, and inversely proportional
to the capacity of the conductor.
The capacity of a conductor may be increased by placing close to it another
conductor which is kept at zero potential. Such an arrangement is called a
condenser. The Leyden jar (see _Leyden Jar_) is a well-known example of a
condenser. The capacity depends not merely on the dimensions of the
conductors and the distance between them, but also upon the nature of the
dielectric separating them. The ratio of the capacity of a condenser with a
given dielectric to the capacity it would have with an air dielectric, is
called the _specific inductive capacity_ of the dielectric. Numerically the
specific inductive capacity of a dielectric is equal to the dielectric
constant already mentioned.
[Illustration: Fig. 5.--Electrophorus]
[Illustration: Fig. 6.--Wimshurst Machine]
In the experimental investigation of electrostatic phenomena it is
convenient to have appliances which will supply charges as they are
required. The simplest appliance of this kind is the electrophorus, which
consists of a disc of ebonite or other suitable material with a metallic
base, and a metal disc of slightly smaller diameter having an insulating
handle attached at right angles to its surface (see fig. 5). To use the
electrophorus, the ebonite is given a negative charge by striking it with
fur or flannel. The metal disc is then placed on top of the ebonite plate.
Since the ebonite is an insulator, no general neutralization of the
positive induced charge on the lower side of the metal disc can take place.
The negative charge on the upper surface of the metal disc is then
neutralized by touching with the finger. The disc is thus left positively
charged. The disc is then lifted by the insulating handle, and the charge
utilized as required. Theoretically speaking, this process may be repeated
continuously without affecting the original charge on the ebonite plate,
but in practice the ebonite has to be re-excited from time to time on
account of the loss of charge by leakage. More elaborate appliances of many
different forms have been used, but the only one of these _electric
machines_, as they are called, which is now commonly employed is the
_Wimshurst machine_. This machine consists of two circular plates of glass
or ebonite carrying equal even numbers of tinfoil sectors symmetrically
placed on their outer surfaces. A pair of brass arms carrying wire brushes,
which simultaneously make contact with diametrically opposite sectors on
each plate, is so arranged as to lie at an angle of about 45deg to the
horizontal, and to be at right angles to one another. A pair of combs is
placed at each end of the horizontal diameter of the plates, so that the
sectors pass close to the teeth of these combs. The combs serve as
collectors, and are connected one pair to the positive pole, and the other
pair to the negative pole of the machine. The general appearance of the
machine is shown in fig. 6. The machine acts on the induction principle,
and if kept warm and dry is self-exciting.
The _electroscope_ is a simple piece of apparatus for detecting the
presence of an electric charge, determining its sign (positive or
negative), and making a very rough comparative estimate of its potential.
It consists of a pair of strips of gold-leaf attached to a brass rod
terminating in a brass cap. The whole is enclosed in a glass case, or a
case having glass sides. The base is made of conducting material. The sides
of the case are coated internally with tinfoil (or two rods connected to
the base project upwards to the level of the gold-leaf strips). The general
appearance of one form of electroscope is shown in fig. 7. The gold-leaf
strips, the brass rod, and the cap must be carefully insulated. When a
charged body is brought near the electroscope the leaves become charged
similarly by induction. The repulsion due to the similar charges causes the
leaves to diverge.
If the cap be touched with the finger, the charge on the leaves is
neutralized, and the leaves collapse. On removing the charged body the
leaves diverge again, owing to the spreading of the charge on the cap,
which was held by the inducing charge, over the whole conductor, including
the leaves. The electroscope is thus charged by induction. It may also be
charged by conduction, i.e. by the direct transfer of a charge to the
electroscope. When we know the kind of charge, positive or negative, which
has been given to the electroscope, an unknown charge can be tested. If the
approach of the unknown charge causes a further divergence of the leaves,
then it is of the same kind as that with which the electroscope is charged.
[Illustration: Fig. 7.--Electroscope]
When accurate quantitative measurements have to be made, an instrument
called an _electrometer_ is used. This instrument, the development of which
is due chiefly to Lord Kelvin, is capable of making accurate measurements
of electrostatic potential differences down to quite low values.
Essentially an electrometer consists of a light suspended conductor which
moves within four fixed quadrants. Opposite pairs of these quadrants are
connected together, one pair to one terminal, and the other pair to the
other terminal of the instrument. The P.D. to be measured is applied at
these terminals. The suspended conductor or 'needle' is charged to a
definite high potential, and the deflection produced is observed from the
movement of a spot of light reflected from a mirror attached to the
suspending fibre. In this case the deflection is proportional to the P.D.
between the quadrants. For measuring a high P.D., the needle may be
connected to one pair of quadrants. With such an arrangement the instrument
is less sensitive, and the deflection is proportional to the square of the
P.D. between the quadrants.
_Current Electricity._ The phenomena connected with the flow of electricity
through a conductor come under this heading. Such a flow of electricity
will take place if by some means the ends of the conductor are maintained
at different potentials. An _electric current_ is then said to exist in the
conductor. The difference of potential may be maintained by chemical action
(see _Daniell's Cell_; _Electric Battery_), by electro-dynamic action (see
_Generator_), or by heat action (see _Thermo-electricity_). The magnitude
of the current which will flow when a steady P.D. is maintained between the
ends of the conductor is determined by what is called the electrical
_resistance_ of the conductor. The resistance R is defined as the ratio of
the applied potential difference V to the current I produced, i.e. R = V/I.
This is a partial expression of Ohm's Law for the Electric Circuit, which
in its most general form states that the current which flows at any instant
in an electric circuit is equal to the algebraic sum of the electromotive
forces existing in the circuit at that instant, divided by the total
resistance in the circuit at that instant (see _Electromotive Force_).
For the particular case where the algebraic sum E of the electromotive
forces is steady, and the total resistance R is not varying, we have I =
E/R. This is the form which applies to steady direct currents. If the
current is changing (whether alternating or merely varying in value),
varying E.M.F.'s, in addition to the applied E.M.F., exist in the circuit,
and the above expression no longer holds good.
The resistance of a conductor depends on its material, and varies directly
as the length, and inversely as the cross-section of the conductor. Thus R
= [rho](l/A), where [rho] is the specific resistance of the material, l the
length of the conductor, and A the cross-sectional area of the conductor.
The _specific resistance_ is the resistance between opposite faces of a
unit cube of the material at a definite temperature (usually 0deg C.). The
resistance of a conductor varies to a greater or less extent with variation
of temperature.
For pure metals the resistance increases considerably with increase of
temperature. With certain alloys the change is so slight as to be
negligible. In some alloys, and in carbon and insulating materials, the
resistance falls with increase of temperature.
_Measurement of Resistance._--Low resistances can most conveniently be
measured by a fall of potential method, based on the relationship R = V/I.
The current may be read by an ammeter, and the potential difference by a
low-reading voltmeter (see _Electrical Measuring Instruments_). Where
greater accuracy is required, a constant current is sent through the
resistance to be measured, and also through a known standard resistance of
about the same value. A sensitive galvanometer (see _Galvanometer_) is used
to compare the P.D. across the unknown resistance with that across the
standard. Since the current is the same through both, the resistances will
be proportional to the galvanometer deflections, and from the known value
of the standard resistance the value of the unknown resistance can be
calculated. Resistances of moderate value are best measured by a Wheatstone
Bridge, or one of its modifications (see _Wheatstone Bridge_).
A substitution method is more suitable for high resistances. A galvanometer
is connected in series with a standard high resistance and a steady source
of E.M.F. The deflection is noted. The unknown resistance is now
substituted for the standard, and the new deflection noted. Provided the
resistance of the galvanometer and other parts of the circuit is negligible
in comparison with the resistance to be measured, the resistances are
inversely as the deflections. The unknown resistance is, therefore, equal
to the ratio of the first to the second deflection multiplied by the value
of the standard resistance. For insulation tests on installations,
direct-reading instruments are frequently used (see _Ohmmeter_).
_Effects of an Electric Current._--When a current flows in a conductor, the
temperature of the conductor is raised. This is due to the power dissipated
on account of the resistance of the conductor. The power dissipated is
equal to I^2R watts, and by giving suitable values to I and R any required
amount of heat per second can be obtained. This _heating effect_ of the
current is made use of in electric lighting, electric heating and cooking,
in electric furnaces, and in certain electro-medical appliances.
If a magnetic needle is brought near a conductor carrying a current, it
will be found to be deflected. This is due to the magnetic field, which is
always associated with an electric current. This _electro-magnetic effect_
is of the utmost practical importance (see _Electro-magnetism_;
_Generator_; _Electric Motors_).
When a current is passed through a conducting liquid, such as a solution of
a metallic salt or a salt in a fused state, chemical action takes place.
The behaviour of such a conductor is entirely different from that of a
metallic conductor, since a current can flow in it only if chemical
dissociation takes place (see _Electrolysis_).
Practical use of electrolysis is made in electroplating, the production of
electrotype blocks for printing, the refining of copper, and the production
of metallic sodium and potassium. Electrolysis is also used as a means of
storing electrical energy in a chemical form (see _Secondary Cell_).
An electric current may be constant in direction (_direct current_), or may
alternate in direction with a certain frequency (_alternating current_).
Alternating currents have advantages for the transmission of large amounts
of power over considerable distances (see _Electric Power Transmission and
Distribution_), and may be used for electric lighting and the operation of
electro-dynamic machines and apparatus (see _Electric Motors_).
BIBLIOGRAPHY: B. Kolbe, _Introduction to Electricity_; S. P. Thompson,
_Elementary Lessons in Electricity_; Starling, _Electricity and Magnetism_;
Poynting and Thomson, _Electricity_; W. E. Ayrton, _Practical Electricity_;
C. R. Gibson, _Electricity of To-day_; Clerk-Maxwell, _Electricity and
Magnetism_; E. E. Brooks and A. W. Poyser, _Magnetism and Electricity_.
ELECTRIC LIGHT, a light obtained by the conversion of electric energy into
light energy. The usual method is to heat some material to incandescence by
passing an electric current through it. The material may be carbon (arc
lamps), tungsten wire (all modern incandescent lamps), mercury vapour
(mercury vapour lamps), or volatilized metallic salts (flame arc lamps).
Other materials have been used, such as zirconium, yttrium, and thorium
oxides, and osmium and tantalum among the metals, but they have been
displaced entirely by the materials mentioned above.
Ordinary arc lamps, and even flame arc lamps, are being displaced by the
modern high-candle-power gas-filled tungsten lamp. Flame arc lamps have a
high efficiency, and are still largely used for street lighting, but the
cost of the frequent trimming required, even in lamps of the magazine type,
gives the gas-filled lamp an advantage over them. Lamps of the mercury
vapour class have a high efficiency, and the light has a high actinic value
which is valuable for certain photographic processes, but the absence of
the red and orange part of the spectrum gives the light a
characteristically ghastly effect which limits the use of this type of
lamp.
_The Carbon Arc._--Although the arc lamp has fallen into disuse, the carbon
arc is still extensively employed for projection work, as in cinema
projectors and in searchlights. The action of the carbon arc is as follows:
If a potential difference of about 50 volts is maintained between a pair of
carbon rods, and the tips of the rods are momentarily brought into contact
and then separated by a short distance, then the current is maintained by
an arc across the gap. The temperature of the positive tip rises to about
4000deg C., and the tip itself soon becomes hollowed, forming what is
called the _positive crater_.
[Illustration: Fig. 1.--Positive and Negative Carbons]
The illustration below represents the two carbons of the arc light as they
appear when cold, the positive carbon being marked + and the negative -.
The central figure is a magnified representation such as can be obtained by
throwing an image of the burning carbons on a screen by means of a lens. In
fig. 1 the upper rod is the positive one, and the hollowed shape of the tip
is clearly shown. The negative tip becomes roughly pointed in shape, and
its temperature is about half that of the positive crater.
The positive crater has an extremely high intrinsic brilliancy, and nearly
the whole of the light is emitted from its surface, the negative tip and
the arc itself contributing very little. In order to stabilize the arc, a
series resistance of a few ohms is necessary. The carbons gradually burn
away, the rate of consumption of the positive carbon being about twice that
of the negative. It is, therefore, necessary to 'feed' the carbons towards
one another. This may be done automatically by the action of a pair of
solenoids, one carrying the current which passes through the arc, the other
carrying a current proportional to the potential difference across the arc.
These solenoids, by means of a suitable mechanism, act in opposition, the
current solenoid separating the carbons, and the potential difference
solenoid bringing them closer together. The actions balance one another
when the arc is of the correct length.
Such an arrangement also serves to strike the arc when the supply is
switched on. In order to prevent the arc from wandering round the carbons,
the positive carbon is cored, and sometimes the negative carbon also. The
core consists of purer softer carbon of lower resistance, and the arc
remains centrally placed.
_Flame Arc Lamps._--The carbon arc principle is modified in these lamps, so
that the arc itself supplies nearly the whole of the light. The arc is made
highly luminous by impregnating the carbons with metallic salts, which are
volatilized and become incandescent in the arc. Their presence also lowers
the resistance of the arc, so that its length can be greatly increased.
The tendency of the arc to wander is also increased, so that cored carbons
are essential, and their diameter must be made as small as possible. These
thin carbons burn away quickly, so that they must be made proportionately
longer for the same time of burning. In order to reduce their resistance a
soft-metal inner core is used. The carbons, instead of being placed one
above the other, are inclined at a small angle with the arc between their
lower ends. The arc is made as large as possible by the action of a small
electromagnet placed just above the gap.
The feeding mechanism is similar in principle to that used for ordinary
carbon arcs. For street lighting, lamps of the magazine type are used. In
these lamps a number of pairs of carbons is placed in the magazine, and as
each carbon is used up, a new one automatically takes its place.