Page:Popular Science Monthly Volume 80.djvu/83

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79
A FLASH OF LIGHTNING

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.