L I G L I G 633 current alone, as this is the same for all the lamps, and might be maintained constant by the adjustment of any one only of the lamps. When lamps are burned in series, it is essential that the difference of potential shall be an clement in the control. This is done by using an electro magnet bound by fine wire so as to have a resistance of some hundreds of ohms, and connecting it to the two sides of the arc. In the Siemens differential lamp, and in some others, a potential or shunt coil and a current coil oppose each other ; as the arc lengthens the current becomes less, and the potential greater, each acting to cause the carbons to approach. It will be seen that the possible combinations of mechanisms and electromagnets for adjusting an electric arc are endless; and so also are the patents for such com binations. 1 When an alternate current is used for an elec tric arc, the phenomena are much more complicated, owing to the difference of potential being a discontinuous function of the time. The difference of potential will be (say) 40 volts in one direction for a certain fraction less than half of the periodic time of the current; the current then entirely ceases, generally for a finite time, and is then reversed with a sudden reversal of difference of potential. 2 The work done in the arc is measured by the time integral of the product of difference of potential and current passing. A knowledge of neither the mean strength of the current, nor of the difference of potential, nor of both, gives the means of ascertaining the work done in an arc with alternate current. The only satisfactory electrical method is the quadrant electrometer suitably connected, and this is open to the objection that a considerable resistance must be introduced into the circuit. Electric Light Jfeasiirements. Under this head we con tent 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 1 See for descriptions of various arc lamps : BROCKIE : Engineering, xxxi. 93 ; Engineer, xlix. 268 ; Tel. Jour., viii. 11-1 ; Electrician, iv. 232. BRUSH : Engineering, xxxi. 55, 85, 123 ; Engineer, Ii. 15 ; Tel. Jour., vii. 21 ; Electrician, iii. 87 ; Fontaine, 45. CANCE : Engineering, xxxiii. 30. CROMPTON : Engineering, xxxii. 205 ; Engineer, xlix. 323; Tel. Jour., viii. 131; Electrician, iv. 273, vii. 229. DE MERSANNE : Engineering, xxxii. 647, 650 ; Shoolbred, 34. FONTAINE : Fontaine, 70. FOUCAULT and DUBOSCQ : Fontaine, 48 ; Schellen, 211. GAIFFE : Fontaine, 36. GORDON: Tel. Jour., iii. 397. GRAMME: Tel. Jour., ix. 250; Engineering, xxxiii. 172. G rLCHER : Engineer, Iii. 343; Tel. Jour., ix. 464; Electrician, vii. 373. HEDOE: Engineering, xxxiii. 393; Electrician, iv. 40, vii. Engineer, xlvii. 167; Tel. Jour., vii. 198; Electrician, ii. 255. LACASSAGNE and THIERS : Fontaine, 28. LONTIN : Shoolbred, 33 ; Fontaine. 59. MACKENZIE : Engineering, xxxi. 38. MAXIM : Tel. Jour., viii. 417, ix. 144 ; Fontaine, 69. MOLERA and CEBRIAN : Tel. Jour., vii. 231. ORME : Tel. Jour., vii. 184. PILSEN : Engineer ing, xxxi. 514, xxxiii. 152; Tel. Jour., viii. 419. RAPIEFF : Engineering, xxvii. 55 ; Tel. Jour. , vii. 60 ; Fontaine, 22 ; Shool 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 ; Slioolbrod, 33. SOLEIL : Engineering, xxxii. 453. STEWART: Tel. Jour., viii. 80, 115. THOMSON and HOUSTAN : Engineer, xlvi. 295 ; Electrician, i. 282 ; Fontaine, 67. TCHIKO- LKFF : Electrician, v. 80. WALLACE-FARMER : Engineer, xlvi. 295 ; Shoolbred, 30 ; Fontaine, 33 ; Purport from the Select Committee on Electric Lighting, 24(5. WESTON : Engineering, xxxii. 42; Elec trician, viii. 246. 2 Joubert, Journal d Physique, ix. 297. 3 Abney, Proc. Roy. Koc., 1878. measurable in terms o.. another mixture in which the pro portions 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. 4 Neither of these considerations applies in the same degree to in candescent lamps. (j. HO.) LIGHTNING. See METEOROLOGY and THUNDER STORMS. LIGHTNING CONDUCTOR, or LIGHTXING ROD (Paratonnerre, Blitz-al>hiter is the name usually given to apparatus designed to protect buildings or ships from the destrustive effects of lightning. The title, alike in English, French, and German, is misleading ; for, when properly constructed, lightning rods serve rather to prevent the occurrence than to ward off the effects of a flash of light ning. Damping the enemy s powder would be a most efficient precaution against cannon-shot, but it w r ould be very inappropriately termed fortification. When a con ductor charged with electricity is brought near to another conductor connected with fche earth, it induces on it a charge of the opposite kind of electricity. The result is an attractive force which tends to bring the conductors nearer to one another, and to augment the electric density on their opposed surfaces. When the density is sufficiently great, there is rupture of the dielectric (air) between the conductors, and the disruptive discharge takes place as an electric spark. If one of the conductors have projecting points or angles, the electric density is usually much greater at such places than over the rest of the surface. But, though the density is great at such places, the charge on them is usually small, and the discharge takes place in an almost continuous manner by a brush or glow. When, for instance, a large conductor, connected with an electric machine, is giving a rapid succession of bright sparks to a ball connected with the ground, the sparks cease as soon as a pointed wire, connected with the ground, is held in the vicinity of the conductor. No discharge is heard, but in the dark a faint glow is seen at the end of the wire, which continues as long as the machine is turned. Re move the wire and the sparks instantly recommence. This glow is known to sailors as St Elmo s (San Telmo s) fire, in old days Castor and Pollux (Plin., //. N., ii. 37). Sup pose now one of the conductors to be a thundercloud, the other the surface of the earth, the discharge will usually take place between the places of greatest surface density ; and it will in general be the more gradual as these are more pointed, and of less capacity. Hence Franklin s idea of furnishing buildings or other prominent objects with a projecting metal spike well connected with the ground, for the purpose of preventing a lightning discharge by substi tuting for it what is practically a continuous electric current. To effect this object thoroughly, only three things are necessary: (1) the points should so project from the building or ship to be protected as to prevent any great development of electric density elsewhere than on them selves ; (2) they should be effectually connected with the earth ; (3) the connecting rod ought to be so good a con ductor as not to be injured even by a powerful electric discharge. The first of these conditions is realized by making the rod branch out to all the salient portions of the building or ship, and furnishing it with points projecting beyond each of them. No general rule can be laid down as to the extent of the region protected by a single point, though it may usually be assumed with safety that the region extends throughout a vertical cone whose vertex is at the point, 4 Allard, Memoire sur les phares llectriqucs, p. 13, Paris, 1881 ; Proc. Inst. C. E., Ivii. 130 ; Shoolbred, 59. xlY. 80
