[1] Gilbert's work, _On the Magnet, Magnetic Bodies and the Great
Magnet, the Earth_, has been translated from the rare folio Latin
edition of 1600, but otherwise reproduced in its original form by the
chief members of the Gilbert Club of England, with a series of
valuable notes by Prof. S.P. Thompson (London, 1900). See also _The
Electrician_, February 21, 1902.
[2] See _The Intellectual Rise in Electricity_, ch. x., by Park
Benjamin (London, 1895).
[3] See Sir Oliver Lodge, "Lightning, Lightning Conductors and
Lightning Protectors," _Journ. Inst. Elec. Eng._ (1889), 18, p. 386,
and the discussion on the subject in the same volume; also the book
by the same author on _Lightning Conductors and Lightning Guards_
(London, 1892).
[4] _The Electrical Researches of the Hon. Henry Cavendish
1771-1781_, edited from the original manuscripts by J. Clerk Maxwell,
F.R.S. (Cambridge, 1879).
[5] In 1878 Clerk Maxwell repeated Cavendish's experiments with
improved apparatus and the employment of a Kelvin quadrant
electrometer as a means of detecting the absence of charge on the
inner conductor after it had been connected to the outer case, and
was thus able to show that if the law of electric attraction varies
inversely as the nth power of the distance, then the exponent n must
have a value of 2 [+-] 1/21600. See Cavendish's _Electrical
Researches_, p. 419.
[6] Modern researches have shown that the loss of charge is in fact
dependent upon the ionization of the air, and that, provided the
atmospheric moisture is prevented from condensing on the insulating
supports, water vapour in the air does not _per se_ bestow on it
conductance for electricity.
[7] Faraday discussed the chemical theory of the pile and arguments
in support of it in the 8th and 16th series of his _Experimental
Researches on Electricity_. De la Rive reviews the subject in his
large _Treatise on Electricity and Magnetism_, vol. ii. ch. iii. The
writer made a contribution to the discussion in 1874 in a paper on
"The Contact Theory of the Galvanic Cell," _Phil. Mag._, 1874, 47, p.
401. Sir Oliver Lodge reviewed the whole position in a paper in 1885,
"On the Seat of the Electromotive Force in a Voltaic Cell," _Journ.
Inst. Elec. Eng._, 1885, 14, p. 186.
[8] "Memoire sur la theorie mathematique des phenomenes
electrodynamiques," _Memoires de l'institut_, 1820, 6; see also _Ann.
de Chim._, 1820, 15.
[9] See M. Faraday, "On some new Electro-Magnetical Motions and on
the Theory of Magnetism," _Quarterly Journal of Science_, 1822, 12,
p. 74; or _Experimental Researches on Electricity_, vol. ii. p. 127.
[10] Amongst the most important of Faraday's quantitative researches
must be included the ingenious and convincing proofs he provided that
the production of any quantity of electricity of one sign is always
accompanied by the production of an equal quantity of electricity of
the opposite sign. See _Experimental Researches on Electricity_, vol.
i. S 1177.
[11] In this connexion the work of George Green (1793-1841) must not
be forgotten. Green's _Essay on the Application of Mathematical
Analysis to the Theories of Electricity and Magnetism_, published in
1828, contains the first exposition of the theory of potential. An
important theorem contained in it is known as Green's theorem, and is
of great value.
[12] See also his _Submarine Telegraphs_ (London, 1898).
[13] The quantitative study of electrical phenomena has been
enormously assisted by the establishment of the absolute system of
electrical measurement due originally to Gauss and Weber. The British
Association for the advancement of science appointed in 1861 a
committee on electrical units, which made its first report in 1862
and has existed ever since. In this work Lord Kelvin took a leading
part. The popularization of the system was greatly assisted by the
publication by Prof. J.D. Everett of _The C.G.S. System of Units_
(London, 1891).
[14] The first paper in which Maxwell began to translate Faraday's
conceptions into mathematical language was "On Faraday's Lines of
Force," read to the Cambridge Philosophical Society on the 10th of
December 1855 and the 11th of February 1856. See Maxwell's _Collected
Scientific Papers_, i. 155.
[15] _A Treatise on Electricity and Magnetism_ (2 vols.), by James
Clerk Maxwell, sometime professor of experimental physics in the
university of Cambridge. A second edition was edited by Sir W.D.
Niven in 1881 and a third by Prof. Sir J.J. Thomson in 1891.
[16] H. von Helmholtz, "On the Modern Development of Faraday's
Conception of Electricity," _Journ. Chem. Soc._, 1881, 39, p. 277.
[17] See Maxwell's _Electricity and Magnetism_, vol. i. p. 350 (2nd
ed., 1881).
[18] "On the Physical Units of Nature," _Phil. Mag._, 1881, [5], 11,
p. 381. Also _Trans. Roy. Soc._ (Dublin, 1891), 4, p. 583.
[19] See Sir W. Thomson, _Proc. Roy. Soc. Lond._, 1856, 8, p. 152; or
Maxwell, _Elect. and Mag._, vol. ii. p. 831.
[20] See Lord Rayleigh, _Proc. Roy. Soc. Lond._, 1884, 37, p. 146;
Gordon, _Phil. Trans._, 1877, 167, p. 1; H. Becquerel, _Ann. Chim.
Phys._, 1882, [3], 27, p. 312.
[21] Perkin's Papers are to be found in the _Journ. Chem. Soc.
Lond._, 1884, p. 421; 1886, p. 177; 1888, p. 561; 1889, p. 680; 1891,
p. 981; 1892, p. 800; 1893, p. 75.
[22] _The Work of Hertz_ (London, 1894).
[23] _L'Ottica delle oscillazioni elettriche_ (Bologna, 1897).
[24] _Les Oscillations electriques_ (Paris, 1894).
[25] _Recent Researches in Electricity and Magnetism_ (Oxford, 1892).
[26] See J.J. Thomson, _Proc. Roy. Inst. Lond._, 1897, 15, p. 419;
also _Phil. Mag._, 1899, [5], 48, p. 547.
[27] Later results show that the mass of a hydrogen atom is not far
from 1.3 X 10^-24 gramme and that the unit atomic charge or natural
unit of electricity is 1.3 X 10^-20 of an electromagnetic C.G.S.
unit. The mass of the electron or corpuscle is 7.0 X 10^-28 gramme
and its diameter is 3 X 10^-13 centimetre. The diameter of a chemical
atom is of the order of 10^-7 centimetre.
See H.A. Lorentz, "The Electron Theory," _Elektrotechnische
Zeitschrift_, 1905, 26, p. 584; or _Science Abstracts_, 1905, 8, A,
p. 603.
[28] See J.J. Thomson, _Electricity and Matter_ (London, 1904).
ELECTRICITY SUPPLY. I. _General Principles._--The improvements made in
the dynamo and electric motor between 1870 and 1880 and also in the
details of the arc and incandescent electric lamp towards the close of
that decade, induced engineers to turn their attention to the question
of the private and public supply of electric current for the purpose of
lighting and power. T.A. Edison[1] and St G. Lane Fox[2] were among the
first to see the possibilities and advantages of public electric supply,
and to devise plans for its practical establishment. If a supply of
electric current has to be furnished to a building the option exists in
many cases of drawing from a public supply or of generating it by a
private plant.
_Private Plants._--In spite of a great amount of ingenuity devoted to
the development of the primary battery and the thermopile, no means of
generation of large currents can compete in economy with the dynamo.
Hence a private electric generating plant involves the erection of a
dynamo which may be driven either by a steam, gas or oil engine, or by
power obtained by means of a turbine from a low or high fall of water.
It may be either directly coupled to the motor, or driven by a belt; and
it may be either a continuous-current machine or an alternator, and if
the latter, either single-phase or polyphase. The convenience of being
able to employ storage batteries in connexion with a private-supply
system is so great that unless power has to be transmitted long
distances, the invariable rule is to employ a continuous-current dynamo.
Where space is valuable this is always coupled direct to the motor; and
if a steam-engine is employed, an enclosed engine is most cleanly and
compact. Where coal or heating gas is available, a gas-engine is
exceedingly convenient, since it requires little attention. Where coal
gas is not available, a Dowson gas-producer can be employed. The
oil-engine has been so improved that it is extensively used in
combination with a direct-coupled or belt-driven dynamo and thus forms a
favourite and easily-managed plant for private electric lighting. Lead
storage cells, however, as at present made, when charged by a
steam-driven dynamo deteriorate less rapidly than when an oil-engine is
employed, the reason being that the charging current is more irregular
in the latter case, since the single cylinder oil-engine only makes an
impulse every other revolution. In connexion with the generator, it is
almost the invariable custom to put down a secondary battery of storage
cells, to enable the supply to be given after the engine has stopped.
This is necessary, not only as a security for the continuity of supply,
but because otherwise the costs of labour in running the engine night
and day become excessive. The storage battery gives its supply
automatically, but the dynamo and engine require incessant skilled
attendance. If the building to be lighted is at some distance from the
engine-house the battery should be placed in the basement of the
building, and underground or overhead conductors, to convey the charging
current, brought to it from the dynamo.
It is usual, in the case of electric lighting installations, to reckon
all lamps in their equivalent number of 8 candle power (c.p.)
incandescent lamps. In lighting a private house or building, the first
thing to be done is to settle the total number of incandescent lamps and
their size, whether 32 c.p., 16 c.p. or 8 c.p. Lamps of 5 c.p. can be
used with advantage in small bedrooms and passages. Each candle-power in
the case of a carbon filament lamp can be taken as equivalent to 3.5
watts, or the 8 c.p. lamp as equal to 30 watts, the 16 c.p. lamp to 60
watts, and so on. In the case of metallic filament lamps about 1.0 or
1.25 watts. Hence if the equivalent of 100 carbon filament 8 c.p. lamps
is required in a building the maximum electric power-supply available
must be 3000 watts or 3 kilowatts. The next matter to consider is the
pressure of supply. If the battery can be in a position near the
building to be lighted, it is best to use 100-volt incandescent lamps
and enclosed arc lamps, which can be worked singly off the 100-volt
circuit. If, however, the lamps are scattered over a wide area, or in
separate buildings somewhat far apart, as in a college or hospital, it
may be better to select 200 volts as the supply pressure. Arc lamps can
then be worked three in series with added resistance. The third step is
to select the size of the dynamo unit and the amount of spare plant. It
is desirable that there should be at least three dynamos, two of which
are capable of taking the whole of the full load, the third being
reserved to replace either of the others when required. The total power
to be absorbed by the lamps and motors (if any) being given, together
with an allowance for extensions, the size of the dynamos can be
settled, and the power of the engines required to drive them determined.
A good rule to follow is that the indicated horse-power (I.H.P.) of the
engine should be double the dynamo full-load output in kilowatts; that
is to say, for a 10-kilowatt dynamo an engine should be capable of
giving 20 indicated (not nominal) H.P. From the I.H.P. of the engine, if
a steam engine, the size of the boiler required for steam production
becomes known. For small plants it is safe to reckon that, including
water waste, boiler capacity should be provided equal to evaporating 40
lb. of water per hour for every I.H.P. of the engine. The locomotive
boiler is a convenient form; but where large amounts of steam are
required, some modification of the Lancashire boiler or the water-tube
boiler is generally adopted. In settling the electromotive force of the
dynamo to be employed, attention must be paid to the question of
charging secondary cells, if these are used. If a secondary battery is
employed in connexion with 100-volt lamps, it is usual to put in 53 or
54 cells. The electromotive force of these cells varies between 2.2 and
1.8 volts as they discharge; hence the above number of cells is
sufficient for maintaining the necessary electromotive force. For
charging, however, it is necessary to provide 2.5 volts per cell, and
the dynamo must therefore have an electromotive force of 135 volts,
_plus_ any voltage required to overcome the fall of potential in the
cable connecting the dynamo with the secondary battery. Supposing this
to be 10 volts, it is safe to install dynamos having an electromotive
force of 150 volts, since by means of resistance in the field circuits
this electromotive force can be lowered to 110 or 115 if it is required
at any time to dispense with the battery. The size of the secondary cell
will be determined by the nature of the supply to be given after the
dynamos have been stopped. It is usual to provide sufficient storage
capacity to run all the lamps for three or four hours without assistance
from the dynamo.
As an example taken from actual practice, the following figures give
the capacity of the plant put down to supply 500 8 c.p. lamps in a
hospital. The dynamos were 15-unit machines, having a full-load
capacity of 100 amperes at 150 volts, each coupled direct to an engine
of 25 H.P.; and a double plant of this description was supplied from
two steel locomotive boilers, each capable of evaporating 800 lb. of
water per hour. One dynamo during the day was used for charging the
storage battery of 54 cells; and at night the discharge from the
cells, together with the current from one of the dynamos, supplied the
lamps until the heaviest part of the load had been taken; after that
the current was drawn from the batteries alone. In working such a
plant it is necessary to have the means of varying the electromotive
force of the dynamo as the charging of the cells proceeds. When they
are nearly exhausted, their electromotive force is less than 2 volts;
but as the charging proceeds, a counter-electromotive force is
gradually built up, and the engineer-in-charge has to raise the
voltage of the dynamo in order to maintain a constant charging
current. This is effected by having the dynamos designed to give
normally the highest E.M.F. required, and then inserting resistance in
their field circuits to reduce it as may be necessary. The space and
attendance required for an oil-engine plant are much less than for a
steam-engine.
_Public Supply._--The methods at present in successful operation for
public electric supply fall into two broad divisions:--(1)
continuous-current systems and (2) alternating-current systems.
Continuous-current systems are either low- or high-pressure. In the
former the current is generated by dynamos at some pressure less than
500 volts, generally about 460 volts, and is supplied to users at half
this pressure by means of a three-wire system (see below) of
distribution, with or without the addition of storage batteries.
Low-pressure continuous supply.
The general arrangements of a low-pressure continuous-current town
supply station are as follows:--If steam is the motive power selected,
it is generated under all the best conditions of economy by a battery of
boilers, and supplied to engines which are now almost invariably coupled
direct, each to its own dynamo, on one common bedplate; a multipolar
dynamo is most usually employed, coupled direct to an enclosed engine.
Parsons or Curtis steam turbines (see STEAM-ENGINE) are frequently
selected, since experience has shown that the costs of oil and
attendance are far less for this type than for the reciprocating engine,
whilst the floor space and, therefore, the building cost are greatly
reduced. In choosing the size of unit to be adopted, the engineer has
need of considerable experience and discretion, and also a full
knowledge of the nature of the public demand for electric current. The
rule is to choose as large units as possible, consistent with security,
because they are proportionately more economical than small ones. The
over-all efficiency of a steam dynamo--that is, the ratio between the
electrical power output, reckoned say in kilowatts, and the I.H.P. of
the engine, reckoned in the same units--is a number which falls rapidly
as the load decreases, but at full load may reach some such value as 80
or 85%. It is common to specify the efficiency, as above defined, which
must be attained by the plant at full-load, and also the efficiencies at
quarter- and half-load which must be reached or exceeded. Hence in the
selection of the size of the units the engineer is guided by the
consideration that whatever units are in use shall be as nearly as
possible fully loaded. If the demand on the station is chiefly for
electric lighting, it varies during the hours of the day and night with
tolerable regularity. If the output of the station, either in amperes or
watts, is represented by the ordinates of a curve, the abscissae of
which represent the hours of the day, this load diagram for a supply
station with lighting load only, is a curve such as is shown in fig. 1,
having a high peak somewhere between 6 and 8 P.M. The area enclosed by
this load-diagram compared with the area of the circumscribing rectangle
is called the _load-factor_ of the station. This varies from day to day
during the year, but on the average for a simple lighting load is not
generally above 10 or 12%, and may be lower. Thus the total output from
the station is only some 10% on an average of that which it would be if
the supply were at all times equal to the maximum demand. Roughly
speaking, therefore, the total output of an electric supply station,
furnishing current chiefly for electric lighting, is at best equal to
about two hours' supply during the day at full load. Hence during the
greater part of the twenty-four hours a large part of the plant is lying
idle. It is usual to provide certain small sets of steam dynamos, called
the daylight machines, for supplying the demand during the day and later
part of the evening, the remainder of the machines being called into
requisition only for a short time. Provision must be made for sufficient
reserve of plant, so that the breakdown of one or more sets will not
cripple the output of the station.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
Three-wire system.
Assuming current to be supplied at about 460 volts by different and
separate steam dynamos, Dy1, Dy2 (fig. 2), the machines are connected
through proper amperemeters and voltmeters with _omnibus bars_, O1, O2,
O3, on a main switchboard, so that any dynamo can be put in connexion or
removed. The switchboard is generally divided into three parts--one
panel for the connexions of the positive feeders, F1, with the positive
terminals of the generators; one for the negative feeders, F3, and
negative generator terminals; while from the third (or middle-wire
panel) proceed an equal number of middle-wire feeders, F2. These sets of
conductors are led out into the district to be supplied with current,
and are there connected into a distributing system, consisting of three
separate insulated conductors, D1, D2, D3, respectively called the
positive, middle and negative distributing mains. The lamps in the
houses, H1, H2, &c., are connected between the middle and negative, and
the middle and positive, mains by smaller supply and service wires. As
far as possible the numbers of lamps installed on the two sides of the
system are kept equal; but since it is not possible to control the
consumption of current, it becomes necessary to provide at the station
two small dynamos called the _balancing machines_, B1, B2, connected
respectively between the middle and positive and the middle and negative
omnibus bars. These machines may have their shafts connected together,
or they may be driven by separate steam dynamos; their function is to
supply the difference in the total current circulating through the whole
of the lamps respectively on the two opposite sides of the middle wire.
If storage batteries are employed in the station, it is usual to install
two complete batteries, S1, S2, which are placed in a separate battery
room and connected between the middle omnibus bar and the two outer
omnibus bars. The extra electromotive force required to charge these
batteries is supplied by two small dynamos b1, b2, called _boosters_. It
is not unusual to join together the two balancing dynamos and the two
boosters on one common bedplate, the shafts being coupled and in line,
and to employ the balancing machines as electromotors to drive the
boosters as required. By the use of _reversible boosters_, such as those
made by the Lancashire Dynamo & Motor Company under the patents of
Turnbull & M^cLeod, having four field windings on the booster magnets
(see _The Electrician_, 1904, p. 303), it is possible to adjust the
relative duty of the dynamos and battery so that the load on the supply
dynamos is always constant. Under these conditions the main engines can
be worked all the time at their maximum steam economy and a smaller
engine plant employed. If the load in the station rises above the fixed
amount, the batteries discharge in parallel with the station dynamos; if
it falls below, the batteries are charged and the station dynamos take
the external load.
[Illustration: From _The Electrician_.