# Lighting, Electric

### current light temperature machine wire machines potential circuit energy armature

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 set forth here ; as they are fully explained in the article 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 conjust at the moment when the direction of the current would change.

The mathematical theory of alternate current machines is comparatively simple.3 Let T 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 let 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 dt ; and the equation of the current will be t Rx = tit ' where x is the current. Now I may be expressed in the form - where A, and /, are constants for the macnine with given excitation of the fixed magnets. Hence 27rs t , , - t 7- + Ex- - A, cos 27rs - dt – - e-Ce 1-'5'=-A, T /(2irs-y tan 27. ST3 - 21-'Sy The term Ce 7 is unimportant except just after closing the circuit. In the Siemens machine N. Joubert states that the only important term is that of longest period. Hence, properly choosing the epoch, we write T x= 27rA 2,7)2 +112 tan - p T Hence we see the current is diminished either by increasing y or increasing B, also that the moment of reversal of current is not coincident with that of no electromotive force, but occurs after that time by an amount depending on the relative magnitudes of 7 and H. This explains in a general way what is known as the lead of the brushes in a continuous current machine. If we wished to apply a commutator to the Siemens alternate current machine for the purpose of producing an external current constant in direction, the change effected by the commutator should occur at an epoch after that of greatest electromotive force, an epoch which, with varying external resistance or varying speed, will depend on the resistance and speed.

The power of the current is Ilx2, and the energy in any considerable time, 0, is or,2,,A21 T2 / 2,7 ya + T ) which shows that most power will be required to drive the machine when In what precedes it has been assumed that the copper wires are the only conducting bodies moving in the magnetic field. In most cases the moving wire coils of these machines have iron cores, the iron being in some eases solid, in others more or less divided. It is found that if such machines are run on open circuit the iron becomes hot, very much hotter than when the circuit of the copper wire is closed; in some cases the phenomenon is so marked that the machine actually takes more to drive it when the circuit is quite open than when the machine is short-circuited. The explanation is that on open circuit currents are induced in the iron cores, but that when the copper coils are closed the current in the latter by its induction diminishes the current in the iron. The effect of currents in the iron cores is not alone' to waste energy and heat the machine ; the current produced is also actually less for a given intensity of field and speed of revolution. The cure of the evil is to subdivide the moving iron as much as possible in directions perpendicular to those in which the current tends to circulate.

Continuous or Direct Current Machines. - It has been shown that to produce a continuous current a commutator is needed. If there is but a single wire in the armature, or if there are more than one, but all are under maximum electromotive force at the same time, the current outside the machine, though always in the same direction, will be far from uniform. This irregularity may be reduced to any extent by multiplying the wires of the armature, giving each its own connexion to the outer circuit, and so placing them that the electromotive force attains a maximum successively in the several circuits. A practically uniform electric current was first commercially produced with the ring armature of Pacinotti as perfected by Gramme. Suppose a straight bar electromagnet surrounded by a coil of copper wire from end to end. Let the electromagnet be bent with the copper wire upon it until its ends meet and it forms an annulus or anchor ring. Let the two ends of the copper wire be connected, so that the iron core is surrounded by an endless copper wire, and you have the Pacinotti or Gramme ring. This ring rotates about its axis of figure between two diametrically opposed magnetic poles of opposite name. The ring may at any instant be supposed divided in halves by a diameter perpendicular to the diameter joining the centre of the poles. Equal and opposite electromotive forces act on the copper wire of the two halves, giving two opposite electric poles half way between the magnetic poles. If electric connexions could be maintained with these two points as the ring revolves, a continuous current would be drawn off. In practice this is only approximated to. The copper wire is divided into a series of equal sections, and at the point of junction of each section with its neighbour a connexion is made with a plate of a commutator, having as many divisions as there are divisions of the copper coil. Collecting, brushes bear upon the commutator plates, which are connected to the coil nearest to the point of maximum potential. Owing to the self-induction and mutual induction of the several coils of the armature, this point is displaced in the direction of rotation when a current is being drawn off, to an extent greater as the current is greater in relation to the strength of the magnetic field. The magnetic field in the Gramme and other continuous dynamo-electric machines may be produced in several ways.' Permanent magnets of steel may be used, as in the smaller machines now made, and in all the earlier machines ; these are frequently called magnetomachines.2 Electromagnets, excited by a current from a smaller dynamo-electric machine, were introduced by Wilde ; these may be described shortly as dynamos with separate exciters. The plan of using the whole current from the armature of the machine itself for exciting the magnets was proposed almost simultaneously by Siemens, Wheatstone, and S. A. Varley.3 For some purposes it is advantageous to divide the current from the armature, sending the greater part through the external circuit, and a smaller portion through the electromagnet, which is then of very much higher resistance, as the electromagnet is a shunt to the external circuit. Machines so arranged are sometimes called shunt dynamos.4 The last two arrangements depend on residual magnetism to initiate the current, and below a certain speed of rotation give no practically useful electromotive force.

In discussing the comparative efficiency of dynamo-machines there are two points to be examined - (l) how much of the power applied is converted into energy of current in the whole circuit, whether external or in the wires of the armature or of the electromagnets, and (2) how much of the power is available outside of the machine. The 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 n2 ; Current round magnets required to produce the field as a.

Thus the work wasted in heating the wire of the electromagnets 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 n3, 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 Light ifeasurements.-Under this head we content ourselves with a warning. A bare statement that an electric arc light is of so many candle power really conveys no accurate information at all. The light from an electric arc differs greatly in colour from that of a candle;3 a given arc light may have three thousand times as much red of a certain wave-length as a standard candle has of the same wave-length, but ten thousand times as much green light. Any one will admit that green light is not measurable in terms of red light; a mixture of red and green is not See for descriptions of various arc lamps :-BROCKIE : Engineering, xxxi. 93 ; Engineer, xlix. 268 ; Tel. Jour., viii. 114 ; Electrician, iv, 232. BRUSH : Engineering, xxxi. 55, 85, 123 ; Engineer, li. 15 ; Tel. Jour., vii. 21 ; Electrician, iii. 87 ; Fontaine, 45. CAECE Engineering, xxxiii. 30. cm: iF1Os : Engineering, xxxii. 205 ; Engineer, xlix. 323 ; Tel. Jour., viii. 131; Electrician, iv. 273, Jour., vii. 301, ix. 73 ; Electrician, iii. 201. IIICKLEY : Tel. Jour., vii. 371. JASPAR : Engineering, xxxii. 645 ; Fontaine, 40. KRUPP: Engineer, xlvii. 167 ; Tel. Jour., vii. 198 ; Electrician, ii. 255. LACASSAGNE and THIERS : Fontaine, 28. LoNTIN Shoolbred, 33 ; Fontaine, 59. MACKENZIE.: Engineering, xxxi. 38. Mnxiai : Jour., viii. 417, ix. 144 ; Fontaine, 69. 1110LERA and CEBRIAN : Tel.

vii. 2.31. ORNIE : Tel. Jour., vii. 184. PILSEN : Engineering, xxxi. 5l4, xxxiii. 152 ; Tel. Jour., viii. 419. PAFIEFF: Engineering, xxvii. 55 ; Tel. Jour., vii. 60 ; Fontaine, 22 ; Shoal-bred, 34 ; Report from the Select Committee on Electric Lighting, 239. SCRIBNER : Tel. Jour., viii. 379. SERRIN Shoolbred, 31 ; Fontaine, 53 ; Schellen, 218. SIEMENS : Engineering, xxxi. 276 ; Tel. Jour., vii. 318, 412, viii. 98 ; Electrician, ii. 52; Schellen, 227 ; Fontaine, 63 ; Shoolbred, 33. SOLEIL ; Engineering, xxxii. 453. STEWART : Tel. Jour., viii. 80, 115. Tuonsos and HOUSTAN: Engineer, xlvi. 295 ; Electrician-, i. 282 ; Fontaine, 67. TCIIIKOLEFF : E1ecerlcian, v. SO. WALLACE-FARMER : Engineer, xlvi. 295 ; Shoolbred. 36 ; Fontaine, 33 ; Report front the Select Committee on Electric Lighting, 246. WESTON : Engineering, xxxii. 42; Electrician, viii. 246.

Joubert, Journal de Physique, ix. 297.

measurable in terms o, another mixture in which the proportions of the colours are wholly different. Again, the intensity of the light obtained from an arc light depends greatly on the direction in which it is viewed.- Neither of these considerations applies in the same degree to incandescent lamps. (J. no.)