February 8, 1900J 



NA TURE 



343 



LETTERS TO THE EDITOR. 



f Tht Eiitor iocs not hold himself responsible for opinions ex- 

 pressei by kis correspondents. Neither can he ^undertake 

 to return, or to correspond with the writers of, rejected 

 minuscripts intended for this or any other part <>/Naturk. 

 Mo notice is taken of anonymous communications.^ 

 The Effects of Lightning upon Electric Lamps. 

 Thk accompanying effects were obtained when attempting 

 [ihotographs ot lightning, and were rather the result of accident 

 than design ; an impending thunderstorm, with a somewhat 

 limited horizon, prevented the camera from being pointed in 

 the direction whence the most numerous of the lightning flashes 

 occurred, without including in the field one or more of the 

 electric arc lamps (Brush) which are the illuminating power of 

 our town. Upon developing the few exposures made, it was 

 noticed that whether or not the picture took in the flash, and in 

 many cases this did not appear at all, there was exhibited upon 

 the films the light of the permanent lamps, and that from them 

 a flow of electricity proceeded towards the ground in an irregular 

 line. Ground or summer lightning, as it is frequently called, 

 produced the same eff"ect upon the lamps, but when neither was 

 present an exposed film only showed a sequence of white dots 

 or perpendicular short strokes in the places occupied by the 

 lamps, as seen by the naked eye at 

 night. The result was .flifiiciently 

 curious to invite further experiments 

 in the same direction, but this could 

 not be done upon the same lines as 

 before, for the electric system varies 

 with the months upon the sea front 

 It Dover. In autumn, winter and 

 pring the usual lighting is con- 

 idered sufficient for all purposes, 

 out during the visitors' season an 

 additional illumination is provided by 

 festoons of smaller glow lights from 

 -landard to standard, with occa- 

 sional cross festoons over the road- 

 way, the whole producing a charming 

 effect at night ; but, as will be seen 

 in No. 6, the detail as to these be- 

 comes confused to any one not 

 acquainted with the relative position 

 of the lamps. 



The first impression these photo- 

 graphs gave me was that the electric 

 discharge in the heavens produced 

 an instantaneous one also from the 

 lamps, and that in this way the 

 circuit was completed : but other 

 causes beyond these must have been 

 in action from the disjointed and 



irregular display from the lamps as opposed to that of the 

 lightning. 



Sir George Stokes, who has kindly interested himself in the 

 matter, appends his views, and I need only add that I hope 

 when a thunderstorm visits a town illuminated by electricity, 

 photographers will, both in daylight and after dark, expose a 

 few plates for the elucidation of the thoughts suggested to us. 

 I say in daylight, for the camera will often record impressions 

 that our eyes cannot see by reason of other external surroundings, 

 as instanced by my noticing upon one occasion with the unaided 

 eye a stream of electricity descending from an arc lamp towards 

 the earth which I should assuredly have never seen had I 

 not learnt from these photographs that such a phenomenon 

 <litl exist. Sydney Wkbb. 



Dover, Octolier 1899. 



I WISH to add a few remarks to Mr. Webb's description of 

 vhe way in which his remarkable photographs were obtained, 

 my object being to point out certain features which seem likely 

 to lead towards an explanation of the discharges which take 

 place, simultaneously with lightning flashes in the sky, in the 

 neighbourhood of the electric lamps. 



Fig. I represents a photograph which was taken looking 

 westwards before the installation from the ornamental glow 

 lamps was set up for the season. The three roundish luminosi- 

 ties represent the normal illumination due to four arc lamps. 



NO 1580, VOL. 61] 



t If these, taken in order along the street, be called Nos. I, 2, 

 \ 3, 4, their order in the picture, from left to right, will be 

 ! I, 2, 4, 3, but Nos. 3, 4 are so nearly in the same line of sight 

 j that the images of their normal liiminosities are blended into 

 I one. The lamps are twenty-one feet above the ground, and the 

 i distance from lamp to lamp is about ninety-two yards. Towards 

 I the upper left is seen a flash of lightning in the sky. Simul- 

 taneously with the flash, electric discharges took place between 

 I the lamps and the ground, which are recorded on the photo- 

 graph. 



It is to be noted that though the lamp-posts were of iron the 

 i discharge did not take that course to the earth, but went in a 

 j curved path which must have been thirty feet or so in length. 

 j Its course led towards the high-tension cable, which ran under- 

 ground along the esplanade ; but whether the cable had any- 

 I thing to do with it, there is not sufficient evidence to show. The 

 j different paths are remarkably similar, almost identical in form. 

 I A striking feature of the discharge is its beaded or stratified 

 j character. The intensity of the discharge and the closeness of 

 I the stratification are both greatest near the lamp, and decrease 

 I as we approach the ground. 



j Another photograph (2a), not here reproduced, was taken 



under the same circumstances, but with a different flash. The 



I general features are the same, but the form of the curves is 



different. Instead oi a curve convex outward, as in Fig. i, we 

 have curves first convex, then concave, then convex again, and 

 as in the former case similar to one another. The negatives of 

 these show a feature which does not appear in the reproduction 

 (Fig. i), nor even in positives taken with sufficient exposure to 

 show the fainter portions of the discharge. In the negative 

 of 2a, the images of lamps 2, 3, 4 show each a pair of short 

 straight dark lines, indicating special luminosity, like two'Ts" 

 parallel to one another and nearly vertical, the base of the right 

 being nearly on a level with the top of the left. No. i, which 

 is much nearer, has too much darkened the negative to show 

 more than a trace of one of the I's. The discharge in 2fl, which 

 shows the stratification, is seen issuing from the top of the right- 

 hand upper 1 in a nearly horizontal direction. The negative of 

 Fig. I shows a similar strong luminosity, only here the I's, if 

 they are distinct, are nearly horizontal, and run one into the 

 other. In this feature, again, we observe as before similarity 

 from lamp to lamp, diflerence from flash tb flash. 



Four photographs were taken looking east along the shore. 

 Three of these are here reproduced (Figs. 2, 3, 4). In one 

 only of these is the flash, which gave rise to the discharges, 

 seen in the field (Fig. 2).' 



The three figures all show one arc lamp which is tolerably 



1 It should be mentioned that in this case, and in this case only, the 

 flash was strengthened by hand on the back of the negative to m.ike it 

 print better, so that the picture of the flash cannot be altogether trusted 

 as to minute details. 



