Popular Science Monthly/Volume 49/July 1896/Photographing Electrical Discharges

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PHOTOGRAPHY plays many important parts in modern science. It assists the astronomer by revealing the existence of thousands and thousands of worlds veiled in the obscurity of immeasurable distance and invisible to the eye even when aided by the most powerful telescope in existence. The chemist has recognized the value of it in registering the belted zones of the spectrum. It aids the meteorologist by placing in his hands permanent records of the dark nimbus and the bright rolling cumulus clouds, the lightning flash, and the automatic registration of the rise and fall of the barometer and thermometer. The microscopist is able to photograph the disease-bearing generations of bacteria, vibrio, and schizomycetes, and to magnify their images a thousandfold for the benefit of the student. It determines the depth of the sea, the direction of currents, and the velocity of projectiles. It detects the spurious bank note or signature, the vibration of suspected bridges, and no traveler's equipment is complete nowadays unless it contains a complete set of photographic apparatus.

These are but a few among the many things that photography does for us. Each year reveals some fresh uses to which it can be placed.

Quite recently the electrician has discovered its value to him in many ways, principally in registering electrical discharges, so that the same may be carefully studied at leisure. At a meeting of the British Association in 1892, Mr. A. A. Campbell Swinton showed some most interesting photographs of electrical discharges which are here reproduced (Figs. 1 and 2). According to an account of them given in the Electrical Review, these figures were

Fig. 1.—A Positive Discharge.

all obtained without the employment of a camera or lens, but produced by merely causing the electrical discharges to take place across the sensitive surface of an ordinary photographic dry plate. Such a plate consists of a sheet of glass coated over with bromide of silver incorporated in a film of gelatin. When single positive or negative discharges are desired, the photographic plate,

Fig. 2.—A Negative Discharge.

the back of which is covered with tin foil, is placed between the two discharging points, and the strength of the discharge is regulated so that, while it shall cover as much of the plate as possible, it shall not spread over the edge. The tin foil performs a double function: it does away with the spark on the back of the plate, which would otherwise occasion a confused figure; and it also assists in causing a uniform spreading out of the discharge. It does not seem to modify the character of the figure produced in any way. It, however, affects its form, since, if the foil be smaller than the plate, the discharge seems to have a great disinclination to extend itself beyond the edge of the foil, but would even curve round to avoid doing this. The arrangement adopted, when it is desired to obtain both positive and negative discharges simultaneously

Fig. 3.—Sextuple Lightning Flash. (By Prof. Zenger.)

on the same plate, and when connecting sparks from positive to negative are desired, is similar, only in these cases the two discharging points are placed on the film side of the plate. Many very beautiful photographs of lightning flashes have been made. They are not at all difficult to get. It is only necessary to wait for a suitable stormy night, point the camera containing the sensitive plate in the direction of the storm, remove the cap, and await the flash. The photographer of lightning flashes is sometimes startled to obtain in his finished picture streaks of lightning, some white and others black, the latter being produced by the flashes which were so actinic as to produce upon the sensitive film the phenomenon known as "reversal." One of the most interesting photographs of lightning flashes that I have seen is that reproduced above. It has been kindly sent to me by Prof. Ch. von Zenger, the renowned meteorologist. It was made on May 20, 1894, when a terrific storm, of short duration fortunately, broke over the town of Prague. The lightning depicted in the photograph struck four houses at once, doing considerable damage. It will be noticed that descending from the clouds are six discharges; one flash can be distinctly traced on its journey to the lightning conductor of the cupola of the Academy of Sciences.

The most interesting part of this photograph is the shadow of the cupola on the wet and foggy heavens. The intensity of the

Fig. 4.—Traces of a Lightning Stroke.

light produced by the lightning is the cause of this peculiar effect, which I believe has never been obtained before or since. It is analogous with the phenomenon commonly known as the Specter of Brocken.

At a meeting of the Academy of Sciences, held a short time ago, Prof. Zenger read a most interesting paper upon the subject of Electricity as a Vortex Motion, in which he endeavors to prove that electrical discharges, no matter what their origin, produce a Fig. 5.—Discharge of a Wimshurst machine. vortex motion on matter lying in the electric field. He makes the following experiment: A spark from a Ruhmkorff coil or a Wimshurst machine is discharged within the receiver of an air pump, under which has been placed a test tube containing diluted ammonia and another containing hydrochloric acid. At the instant of the discharge there will be visible eddies or little whirlwinds, rendered visible by the tiny crystals of ammonium chlorid suspended in the air. In circling about they are condensed into peculiar shaped veins which fall upon the plate of the air pump, arranging themselves in somewhat the same manner, says Zenger, as did the débris from the roofs and trees during the cyclone in the valley of Roux in Switzerland—i. e., they form lines of electric force.

Prof. Zenger sends me some very interesting photographs in support of the vortex theory. Fig. 4 shows the marks left by a stroke of lightning on the wall of a house at Mort, illustrating its Fig. 6.—Discharge of a Ruhmkorff Induction Coil. course from the iron water pipe to the iron hinges of the window, and from thence to the iron sewer grating. For comparison he sends two other photographs (Figs. 5 and G). These are obtained by the discharge of an immense Ruhmkorff induction coil containing one hundred thousand metres of insulated copper wire. Two small triangles of tin foil are glued opposite to one another on a photographic plate, and when dry the whole is

Fig. 7.—Artificial Solar Eclipse.

covered with a light brownish coating of lampblack. After the discharge a white track is found, as shown in the photographs. Both these pictures show evidence of the vortex motion of the lampblack driven off by the discharge, and the same peculiar formation is seen in the lightning flash as in Fig. 4. Between the dark filament of intact lampblack and the borders of the electric tracings there is a multitude of fine helicoidal curves filling the white traces left on the smoked glass. These helicoidal spires proceed to the right and left, and on about the middle of the trace the destruction of the two opposing motions produces the phenomena of discharge, viz., light, heat, sound, and mechanical effects. Zenger claims that these, his experiments, serve to show that the undulatory theory of Herz is very defective, there being no motion in plane waves, nor, as in the case of polarized light, is there any in circular or elliptical waves on a cylindrical surface.

"I could show," writes Prof. Zenger, in a recent letter to the writer, "that the solar phenomena, such as the solar corona and

Fig. 8.—Imitation of the Solar Chromosphere and Corona.

protuberances, are also due to electric discharges continually given off from the sun's surface. Fig. 7 shows the effect of positive discharges from a circular disk of tin foil into the smoked surface of a glass plate. It will be noticed that we have facsimiles of all the forms of solar protuberances. In Fig. 8 we see the effect of a positive discharge from a hollow hemisphere into the smoked surface of a glass plate, on the middle of which was fixed a lump of wax. The effects are analogous to the chromosphere and to the inner and outer layers of the solar corona produced by the superposition of the lines of electric force radiating from different parts of the spherical surface."

Prof. Zenger's photographs are remarkably interesting and are valuable assistants in support of the vortex theory which he advances. I hope at an early date to supplement this article with some still more interesting examples of the photography of electrical discharges.