Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/654

 632 LIGHTING practical sources of loss are friction of bearings, and of the brushes on the commutator, electric currents induced in the iron of the machine, production of heat in the copper wire of the armature due to its resistance, and production of heat in the wire of the electromagnet due to its resistance. There is also a certain loss in sparks upon the commutator. The currents in the iron are reduced by dividing the iron by insulating surfaces perpendicular to the electromotive force tending to produce such currents. The loss by resistance of wire in armature and magnets greatly depends on the dimensions of the machine. For imagine two exactly similar dynamo-electric machines, the one being n times the dimensions of the other, we have the following relations between them, assuming the same magnetic field per square centimetre, and the same speed of rotation : The electric resistances of the several parts are as 1 : n ; The electromotive force of the armature as 7i 2 ; Current round magnets required to produce the field as n. Thus the work wasted in heating the wire of the electro magnets varies as the linear dimensions of the machine. The current which the armature can carry with safety to the insulation will increase more rapidly than the linear dimensions of the machines, but less rapidly than the square of the linear dimensions. If the current vary as the linear dimensions n, the whole electric work done by the machine will vary as its weight n 3, and the work wasted in the coils both of the electromagnets and of the armature will only vary as n, showing a great theoretic advantage in favour of the larger machines. Electric Lamps, Incandescent Lamp. 1 The simplest way of obtaining light from an electric current is by passing it through a considerable resistance in such small compass that the conductor becomes intensely hot. It is of course necessary that the conductor shall be able to endure a very high temperature without injury. Iridium and platinum- iridium wire have been employed, but are too expensive for commercial use. Hitherto the only available sub stance is carbon, in the form of a thread or filament. This carbon must be protected from the air by en closing it in a glass globe from which every trace of air has been removed. An electric current passing through a carbon filament obeys Ohm s law, as through a metallic wire. But in metals the resistance increases as the tem perature rises, in carbon it diminishes. 2 The filament or thread of carbon being enclosed in a vacuous space, the energy of current converted into heat in the filament only leaves it in the shape of radiations. To light economically, it is necessary to heat the filament to such a temperature that the greatest possible proportion of these radiations shall belong to that part of the spectrum to which the eye is sensitive, i.e., to the highest temperature the filament will stand. The fundamental problem of incandescent electric lighting is to produce a carbon thread the substance of which shall permanently stand the highest possible temperature, to make good electrical connexion between the ends of the filament and the conducting wares, and above all to secure that the thread shall be uniform throughout its length, for the current which can be safely used is limited by the weakest point of the filament. Several inventors have recently succeeded in meeting thesa 1 See for descriptions of various incandescent lamps : BROUGHAM and ANDRE: Engineer, xlix. 190; Electrician, iv. 213. EDISON: Engineering, xxxiii. 305, 407; Tel. Jour., viii. 28, x. 440; Electri cian, vii. 362. Fox (ST G. LANE): Engineer, li. 197; Tel. Jour., ix. 97; Electrician, viii. 5. JABLOCHKOFF : Engineering, xxxii. 391. KONN : Electrician, vii. 361 ; Fontaine, 292. MAXIM : Engineering, xxxi. 618. NAPOLI: Electrician, vi. 138. SAWYER-MANN : Engineer, xlvii. 5; Tel. Jour., vii. 7; Electrician, ii. 100. STARR: Electri cian, vii. 361. SWAN : Tel. Jour., viii. 378 ; Electrician, vii. 218. 377. 2 Thomson, Tel. Jour., ix. 378. conditions, but their relative merit and priority cannot be discussed here. 3 Semi-incandescent Lamp. The lamps of Werdennann, Reynier, and Joel are intermediate between arc lamps and incandescent lamps, and present the distinctive advantages of neither. 4 Arc Lights. Sir Humphry Davy discovered that if two pieces of carbon were placed in contact with each other, and the current from a battery of a sufficient number of elements were passed from one piece to the other, the current did not cease when the carbons were slightly parted, but that the current passed across the intervening space, causing an intensely high temperature and consequently brilliant light. The pieces of carbon gradually burned away, the positive carbon being consumed more rapidly than the negative. When an electric current passes through a conducting solid body maintained at a con stant temperature, the difference of potential on the two sides of the body has a constant ratio to the current pass ing through; this constant ratio is known as the electric resistance of the body at its then temperature. No such constant ratio exists in the case of the electric arc. If you increase the current passing between two carbons at a small distance apart, you do not materially change the difference of potential at the two ends of the electric arc. It is, therefore, not strictly appropriate to speak of the resistance of the electric arc ; the appropriate con stant, or approximate constant, for an electric arc is the difference of potential between the tw r o sides of the arc. 5 However near the carbons approach without touching, this does not fall below a certain minimum value, and as the carbons are separated its value increases. In ordinary practice with continuous currents the potential of the electric arc may be taken as ranging from 35 to 45 volts. If the current in amperes be multiplied by the difference of potential in volts, and the product be divided by 74G, w T e have the power used in the arc itself in horse-power, that is, the power effectively used in lighting. The mechanism of an electric lamp has two functions to perform, it has first to bring the carbons into contact and then part them, or simply part them if they are initially in contact when the light is started, or when it is accidentally extinguished (this is called striking the arc) ; it has also to bring the carbons together as they are consumed. The former function is always accomplished by an electro magnet or solenoid. In the electric candles, e.g., those of Jablochkoff, Rapieff, Wilde, or Siemens, the carbons are approximately parallel, and they burn down as does a candle, the arc being forced to the ends of the carbons by the repulsion of the current in the carbons on the electric arc. 6 In the ordinary arc lamps the carbons have their axes in the same line, and their approach or recession must be controlled by the current passing through, by the difference of potential, or by both combined. When the same current passes through a succession of lamps in series, it is clear that the regulation cannot be by the 3 The application of incandescent lighting on a larpe scale has beeu thoroughly worked out in all its details by Edison. For a description of the whole system see Engineering, xxxiii. 226, 250, 305, 407. 4 See for description of various semi-incandescent lamps : JOEL : Tel. Jour., viii. 364 ; Electrician, vi. 293. REYNIER : Electrician, ii. 88 ; Tel. Jour., vii. 335 ; Fontaine, 297 ; Shoolbred, 38. WERDEII- MANN : Engineer, xlvi. 312 ; Fontaine, 301 ; Shoolbred, 39. 5 Edlund, Pogg. Annal., 1867, 1868. 6 See for descriptions of various electric candles : DE MERITENS : Shoolbred, 43. GATEHOUSE : Tel. Jour., viii. 352. HEINKICH r Engineer, xlviii. 413. HICKLEY : Tel. Jour., vii. 229. JABLOCH KOFF: Engineering, xxxii. 251, 300, 326 ; Tel. Jour., vii. 10 ; Shool bred, 41 ; Fontaine, 76 ; Report from the Select Committee on Electric Lighting, 233 ; Schellen, 243. JAMIN : Tel. Jour., viii. 224 ; Electri cian, v. 68; Fontaine, 81. RAPIEFF: Tel. Jour, vii. 43; Shoolbred, 44. WILDE : Tel. Jour., vii. 46 ; Shoolbred, 44; Engineering, xlvii. 385.