Popular Science Monthly/Volume 80/January 1912/A Flash of Lightning




IT is customary to classify lightning discharges into at least two classes. This classification is based on the appearance of the flash. One kind of lightning is called forked lightning and the other sheet lightning. There has been some discussion concerning sheet lightning, it being claimed by some that it is merely an illumination due to a discharge which is hidden from view.

The real fact appears to be that both ends of a lightning flash are usually hidden from view within the two clouds. One of these clouds contains falling drops of water which are overcharged with the negative corpuscles which atoms of all kinds of matter contain when in normal condition. The other cloud contains drops which have less than the normal charge. This cloud has always been said to be positively charged.

The writer has sought to obtain photographic evidence of the conditions within these two clouds, at the instant when the discharge

Fig. 1. The Overcharged Cloud. An inflow of the negative fluid to the main discharge channel, whose end is seen at the middle of the plate.
Fig. 2. The Outflow into the Cloud which has less than its Normal Charge.

occurs. This evidence is presented in the figures, which are reproduced from photographic plates.

Fig. 1 represents in cross-section the cloud which is overcharged with the negative fluid. The cross-section is at right angles to the end of the long flash which connects the two clouds. The discharge lines on this plate resemble a system of rivers and tributaries, which penetrate the cloud. These drainage lines elongate up stream. Some of them are sharply defined. Others, for reasons which will be explained, are seen only in shadowy outline.

Before the flash occurred, the falling drops, which were all highly charged, repelled each other. After the discharge those drops which happened to lie in the path of some one of these tributary discharge lines, have lost their overcharge. There is then an attraction between such drops and those which were slightly outside of these drainage lines, and which are therefore still overcharged. These two groups of drops are intimately commingled, as is shown by the intricate nature of the system of drainage channels. As they continue their fall to earth, they coalesce, and a brief dash of unusually large drops of rain is observed.

The discharge pours through the long hole in the air, in which the conditions are like those which exist in a vacuum tube. The conditions which exist in the cloud at the other end of the flash are shown in Fig. 2.
Fig. 3. An Inflow as in Fig. 1.
Fig. 4. An Outflow as in Fig. 2.

Here we have a representation of the outflow into the cloud which has less than its normal charge. The flash here diffuses into the cloud, and the outer portions of the flash might well be called sheet lightning. The conditions which would bring about the brief dash of large raindrops do not appear to exist at this end of the lightning flash.

Figs. 3 and 4 are similar to Figs. 1 and 2, the discharge being somewhat more violent, or greater in quantity. In Fig. 3, it may be seen that the inflow in some eases begins at isolated points, and progresses inwardly towards inflow lines which are elongating in a direction opposite to that in which the flow is taking place.

If these discharge figures are to be described in the language of the two-fluid theory, Figs. 1 and 3 must be called an outward positive discharge. Figs. 2 and 4 must be called an outward negative discharge. We must say that the outward negative discharge shown in Figs. 2 and 4 came from the cloud represented in Figs. 1 and 3, and that the outward positive discharge, shown in Figs. 1 and 3, came from the cloud represented in Figs. 2 and 4.

Such an explanation seems so essentially absurd in the presence of these photographic plates, that it will not be urged.

In conclusion, however, a confession must be made. The lightning discharge here described was artificially produced. A plate-glass machine, with metal conductors terminating in pin-heads took the place of the long flash of lightning. The pin-heads rested upon the centers of the two photographic films, the plates resting on large sheets of glass. There were small spark-gaps of about half an inch in each line, at the machine terminals. A single spark across these gaps produced a glow over the films around the pin-heads. In order to bring some of the discharge lines down into close proximity to the films, so that they would be sharply defined, copper plates were placed under each photographic plate below the sheet of glass. These copper plates were grounded or, what produces the same result, they were connected with each other. No trace of the discharge can be detected until the photographic plate is developed. With this confession, and with an apology for having misled the reader, the question may be asked, can any one look at Figs. 1 and 3 and believe that they are produced by an outward discharge of positive electricity? These plates tell their own story. They represent an inflow of negative corpuscles, along drainage channels.

Some of the lines are not sharply defined. They are too far above the film. They all present a shadowy appearance, if the condenser action of the copper plates is eliminated.