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 The reader may be referred for an admirable summary of the theories of electricity prior to the advent of the electronic hypothesis to J. J. Thomson’s “Report on Electrical Theories” (Brit. Assoc. Report, 1885), in which he divides electrical theories enunciated during the 19th century into four classes, and summarizes the opinions and theories of A. M. Ampère, H. G. Grassman, C. F. Gauss, W. E. Weber, G. F. B. Riemann, R. J. E. Clausius, F. E. Neumann and H. von Helmholtz.

.—M. Faraday, Experimental Researches in Electricity (3 vols., London, 1839, 1844, 1855); A. A. De la Rive, Treatise on Electricity (3 vols., London, 1853, 1858); J. Clerk Maxwell, A Treatise on Electricity and Magnetism (2 vols., 3rd ed., 1892); id., Scientific Papers (2 vols., edited by Sir W. J. Niven, Cambridge, 1890); H. M. Noad, A Manual of Electricity (2 vols., London, 1855, 1857); J. J. Thomson, Recent Researches in Electricity and Magnetism (Oxford, 1893); id., Conduction of Electricity through Gases (Cambridge, 1903); id., Electricity and Matter (London, 1904); O. Heaviside, Electromagnetic Theory (London, 1893); O. J. Lodge, Modern Views of Electricity (London, 1889); E. Mascart and J. Joubert, A Treatise on Electricity and Magnetism, English trans. by E. Atkinson (2 vols., London, 1883); Park Benjamin, The Intellectual Rise in Electricity (London, 1895); G. C. Foster and A. W. Porter, Electricity and Magnetism (London, 1903); A. Gray, A Treatise on Magnetism and Electricity (London, 1898); H. W. Watson and S. H. Burbury, The Mathematical Theory of Electricity and Magnetism (2 vols., 1885); Lord Kelvin (Sir William Thomson), Mathematical and Physical Papers (3 vols., Cambridge, 1882); Lord Rayleigh, Scientific Papers (4 vols., Cambridge, 1903); A. Winkelmann, Handbuch der Physik, vols. iii. and iv. (Breslau, 1903 and 1905; a mine of wealth for references to original papers on electricity and magnetism from the earliest date up to modern times). For particular information on the modern Electronic theory the reader may consult W. Kaufmann, “The Developments of the Electron Idea.” Physikalische Zeitschrift (1st of Oct. 1901), or The Electrician (1901), 48, p. 95; H. A. Lorentz, The Theory of Electrons (1909); E. E. Fournier d’Albe, The Electron Theory (London, 1906); H. Abraham and P. Langevin, Ions, Electrons, Corpuscles (Paris, 1905); J. A. Fleming, “The Electronic Theory of Electricity,” Popular Science Monthly (May 1902); Sir Oliver J. Lodge, Electrons, or the Nature and Properties of Negative Electricity (London, 1907).

 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 and St G. Lane Fox 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 ℔ 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