Lightning, Thunder and Lightning Conductors

by Gerald Molloy

I can continue to take sparks from the conductor as long as the machine is worked; and it is interesting to observe that these sparks follow an irregular zig-zag course, just as lightning does. The reason is the same in both cases: a discharge between two electrified bodies takes place along the line of least resistance; and, owing to the varying condition of the atmosphere, as well as of the minute particles of matter floating in it, the line of least resistance is almost always a zig-zag line.

What a Flash of Lightning is.—Lightning, then, may be conceived as an electrical discharge, sudden and violent in its character, which takes place, through the atmosphere, between two bodies highly charged with opposite kinds of electricity. Sometimes this electrical discharge passes, as I have said, between a cloud and the earth; sometimes it passes between one cloud and another; sometimes, on a smaller scale, it takes place, between the great mass of a cloud and its outlying fragments.

But, if you ask me in what the discharge itself consists, I am utterly unable to tell you. It is usual to speak and write on this subject as if electricity were a material substance, a very subtle fluid, and as if, at the moment the discharge takes place, this fluid passes like a rapid stream, from the body that is positively electrified to the body that is negatively electrified. But we must always remember that this is only a conventional mode of expression, intended chiefly to assist our conceptions, and to help us to talk about the phenomena. It does not even profess to represent the objective truth. All that we know for certain is this: that immediately before the discharge the two bodies are highly electrified with opposite kinds of electricity; and, that immediately after the discharge, they are found to have returned to their ordinary condition, or, at least, to have become less highly electrified than they were before.

The flash of light that accompanies an electric discharge is often supposed to be the electricity itself, passing from one body to the other. But it is not; it is simply an effect produced by the discharge. Heat is generated by the expenditure of electrical energy, in overcoming the resistance offered by the atmosphere; and this heat is so intense, that it produces a brilliant incandescence along the path of the discharge. When a spark appears, for example, between the conductor of the machine and this brass ball, it can be shown, by very satisfactory evidence, that minute particles of these solid bodies are first converted into vapor, and then made to glow with intense heat. The gases, too, of which the air is composed, and the solid particles floating in the air, are likewise raised to incandescence. So, too, with lightning; the flash of light is due to the intense heat generated by the electrical discharge, and owes its character to the composition and the density of the atmosphere through which the discharge passes.

Duration of a Flash of Lightning.—How long does a flash of lightning last? You are aware, I dare say, that when an impression of light is made on the eye, the impression remains for a sensible interval of time, not less than the tenth of a second, after the source of light has been extinguished or removed. Hence we continue, in fact, to see the light, for at least the tenth of a second, after the light has ceased. Now, if you reflect how brief is the moment for which a flash of lightning is visible, and if you deduct the tenth of a second from that brief moment, you will see, at once, that the period of its actual duration must be very short indeed.

The exact duration of a flash of lightning is a question on which no settled opinion has yet been accepted generally by scientific men. Indeed, the most widely different statements have been made on the subject, quite recently, by the highest authorities, each speaking apparently with unhesitating confidence. Thus, for example, Professor Mascart describes an experiment, which he says was made by Wheatstone, and which showed that a flash of lightning lasts for less than one-thousandth of a second;[2] Professor Everett describes the same experiment, without saying by whom it was made, and gives, as the result, that “the duration of the illumination produced by lightning is certainly less than the ten-thousandth of a second;”[3] Professor Tyndall, in his own picturesque way, tells us that “a flash of lightning cleaves a cloud, appearing and disappearing in less than the hundred-thousandth of a second;”[4] and according to Professor Tait, of Edinburgh, “Wheatstone has shown that lightning certainly lasts less than the millionth of a second.”[5]

Experiments of Professor Rood.—I cannot say which of these statements is best supported by actual observation; for none of the writers I have quoted gives any reference to the original memoir from which his statement is derived. As far as my own reading goes, I have only come across one original record of experiments, made directly on the flash of lightning itself, with a view to determine the period of its duration. These experiments were carried out by Professor Ogden Rood, of Columbia College, New York, between the years 1870 and 1873, and are recorded in the American Journal of Science and Arts.[6]

For the description of his apparatus, and for the details of his observations, I must refer you to the memoir itself; but I may tell you briefly that the results at which he arrived, if they be accepted, must lead to a considerable modification of the views previously entertained on the subject. In the first place, he satisfied himself that what appears to the eye a single flash of lightning is usually, if not always, multiple in its character; consisting, in fact, of a succession of distinct flashes, which follow one another with such rapidity as to make a continuous impression on the retina. Next, he proceeded to measure approximately the duration of these several component flashes; and he found that it varied over a wide range, amounting sometimes to fully the twentieth of a second, and being sometimes less than the sixteen-hundredth of a second.

Wheatstone’s Experiments.—These results are extremely interesting; but we can hardly regard them as finally established, until they have been confirmed by other observers. I may remark, however, that they fit in very well with the experiments made by Professor Wheatstone, many years ago, on the duration of the electric spark, which, as I told you, is a miniature flash of lightning. In these classical experiments, which leave nothing to be desired in point of accuracy, Professor Wheatstone showed that a spark taken directly from a Leyden jar, or a spark taken from the conductor of a powerful electric machine, that is, just such a spark as you have seen here to-day, lasts for less than the millionth of a second.

But he also showed that the duration of the spark is greatly increased, when a resisting wire is introduced into the path of the discharge. Thus, for example, when the discharge from a Leyden jar was made to pass through half a mile of copper wire, with breaks at intervals, the sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀ of a second.[7] Hence we should naturally expect that the period of illumination would be still further increased, in the case of a flash of lightning, where the resistance interposed is enormously greater than in either of the experiments made by Wheatstone.[8]