Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/652

 630 LIGHTING LIGHTING, ELECTRIC. Artificial light is generally produced by raising some body to a high, temperature. If the temperature of a solid body be greater than that of surrounding bodies it parts with some of its energy in the form of radiation. Whilst the temperature is low these radiations are not of a kind to which the eye is sensitive ; they are exclusively radiations less refrangible and of greater wave-length than red light, and may be called infra-red. As the temperature is increased the infra-red radiations increase, but presently there are added radiations which the eye perceives as red light, As the temperature is further increased, the red light increases, and yellow, green, and blue rays are successively thrown off. On pushing the temperature to a still higher point, radiations of a wave-length shorter even than violet light are pro duced, to which the eye is insensitive, but which act strongly on certain chemical substances; these may be called ultra-violet rays. It is thus seen that a very hot body in general throws out rays of various wave length, our eyes, it so happens, being only sensitive to certain of these, viz., those not very long and not very short, and that the hotter the body the more of every kind of radiation will it throw out, but the proportion of short waves to long waves becomes vastly greater as the temperature is increased. The problem of the artificial production of light with economy of energy is the same as that of raising some body to such a temperature that it shall give as large a proportion as possible of those rays which the eye happens to be capable of feeling. For practical purposes this temperature is the highest tempera ture we can produce. As an illustration of the luminous effect of the high temperature produced by converting other forms of energy into heat within a small space, con sider the following statements. 120 cubic feet of 15 candle gas will, if burned in ordinary gas burners, give a light of 360 standard candles for one hour. The heat pro duced by the combustion is equivalent to about GO million foot-pounds. If this gas be burned in a gas-engine, about 8 million foot-pounds of useful work will be done outside the engine, or four horse-power for one hour. This is sufficient to drive an &quot; A &quot; Gramme machine for one hour ; the energy of the current will be about 6,400,000 foot pounds per hour, about half of which, or only 3,200,000 foot-pounds, is converted into radiant energy in the electric arc, but this electric arc will radiate a light of 2000 candles when viewed horizontally, and two or three times as much when viewed from below. Hence 3 million foot-pounds changed to heat in the electric arc may be said roughly to affect our eyes six times as much as 60 million foot-pounds changed to heat in an ordinary gas burner. 1 Owing to the high temperature at which it remains solid, and to its great emissive power, the radiant body used for artificial illumination is nearly always some form of carbon. The consideration of electric lighting naturally divides into two parts the production of suitable electric currents, and the conversion of the energy of such currents into radiations. Although electric lights were first produced from currents generated by batteries, they have only attained commercial importance by the use of machines for converting mechanical energy into electric current. Dynamo-Electric Machines. In the widest sense a dynamo-electric machine may be defined as an apparatus for converting mechanical energy into the energy of electrostatic charge, or mechanical power into its equivalent electric current through a conductor. Under this definition would be included the electrophorus and all frictional machines ; but the term is used in a more restricted sense 1 Proc. Inst. C. E., lii. 69 ; Report from the Select Committee on Electric Lighting, 1879. for those machines which produce electric currents by the motion of conductors in a magnetic field, or by the motion of a magnetic field in the neighbourhood of a conductor. The general laws of electromagnetic induction need not be set forth here ; as they are fully explained in the article ELECTRICITY, they will be assumed in all that follows. Since, if the current in a closed circuit be in one direction when the number of lines of force is increasing, it will be in the opposite direction when they are diminishing, it is clear that the current in each part of the circuit which passes through the magnetic field must be alternate in direction. Hence also the current in the wire outside the machine must also be alternate, unless something of the nature of a commutator be employed to reverse the con nexions of the internal wires, in which the current is induced, and the external circuit. We have then broadly two classes of dynamo-electric machines: the simplest, the alternate current machine, 2 where no commutator is used ; and the continuous current, in which a commutator is used to change the connexions to the external circuit just at the moment when the direction of the current would change. The mathematical theory of alternate current machines is com paratively simple. 3 Let r be the period of the machine, that is, the time taken to move the armature from one position to the next exactly similar position, e.g., in a Siemens alternate current machine of sixteen magnets on each side, one-eighth of the time of revolution ; let 7 be the coefficient of self-induction of the whole circuit, and R the resistance of the whole circuit ; and h t t denote the time at any instant counting from any epoch as initial, and I the magnetic induction at time t multiplied by the number of convolutions. The electromotive force in the circuit at time t will be rfl. dt and the equation of the current will be dr dl 7-j- + Ra: = -5- , at at where x is the current. Now I may be expressed in the form ~r. o t-t, 2i A, sin 2irs - , where A, and t, are constants for the macnine with given excitation of the fixed magnets. Hence ijf 9ir / / -.&quot;&quot;* i T&amp;gt;_, i-OO z 7r 5 I. ._ n - &quot; &quot; 27T.S cos 2irs- tan The term Ce v is unimportant except just after closing the circuit. In the Siemens machine M. Joubert states that the only 2 For descriptions of various alternate current machines, consult the following authorities: ALLIANCE: H. Fontaine, Eclairage d VElec- tricite, Paris, 1879, p. 114 ; Dr H. Schellen, Die magnet- und dynamo- electrischen Maschinen, Cologne, 1879, p. 35. DK MERITENS: Engineer ing, xxviii. 372, xxxii. 356, 380, 392; Tel. Jour., vi. 393; Shoolbred, Electric Lighting, London, 1879, p. 14 ; Electrician, ii. 27. GRAMME: Engineering, xviii. 413, xxvi. 63, xxix. 134, xxxii. 615 ; Tel. Jour., viii. 26, ix. 206 ; Electrician, iv. 176 ; Shoolbred, 25 ; Fontaine, 161 ; Schellen, 176 ; Report from the Select Committee on Electric Lighting, 231. HOLMES: Fontaine, 119 ; Schellen, 35. HOLMES- ALLIANCE: Shoolbred, 13. LACHAUSSI?E: Engineering, xxxii. 465; Tel. Jour., ix. 381. LONTIN: Engineering, xxv. 48, xxviii. 174 ; Shoolbred, 22 ; Fontaine, 171 ; Report from the Select Committee on Electric Lighting, 229 ; Schellen, 167. RAPIEFP : Tel. Jour., viii. 150. SIEMENS: Tel. Jour., vii. 284; Schellen, 315. 3 Journal de Physique, x. 141 ; Joubert, &quot; Theorie des machines a courants alternatifs,&quot; in Phil. Mag., x. 298, 384.