Lightning, Thunder and Lightning Conductors

by Gerald Molloy

Franklin’s Experiment.—Meanwhile Franklin had been waiting, with impatience, for the completion of the tower of Christchurch, in Philadelphia, on which he intended to make the experiment himself. He even collected money, it is said, to hasten on the building. But, notwithstanding his exertions, the progress of the tower was slow; and his active mind, which could ill brook delay, hit upon another expedient, remarkable alike for its simplicity and for its complete success. He constructed a boy’s kite, using, however, a silk pockethandkerchief, instead of paper, that it might not be damaged by rain. To the top of the kite he attached a pointed iron wire about a foot long, and he provided a roll of hempen twine, which he knew to be a conductor of electricity, for flying it. This was the apparatus with which he proposed to explore the nature of a thundercloud.

The thundercloud came late in the afternoon of the fourth of July, 1752, and Franklin sallied out with his kite, accompanied by his son, and taking with him a common door-key and a Leyden jar. The kite was soon high in air, and the philosopher awaited the result of his experiment, standing, with his son, under the lee of a cowshed, partly to protect himself from the rain that was coming, and partly, it is said, to shield himself from the ridicule of passers-by, who, having no sympathy with his philosophical speculations, might be inclined to regard him as a lunatic. To guard against the danger of receiving a flash of lightning through his body, he held the kite by means of a silk ribbon, which was tied to the door-key, the door-key being itself attached to the lower end of the hempen string.

A flash of lightning soon came from the cloud, and a second, and a third; but no sign of electricity could be observed in the kite, or the hempen cord, or the key. Franklin was almost beginning to despair of success, when suddenly he noticed that the little fibres of the cord began to bristle up, just as they would if it were placed near an electric machine in action. He presented the door-key to the knob of the Leyden jar, and a spark passed between them. Presently a shower began to fall; the cord, wetted by the rain, became a better conductor than it had been before, and sparks came more freely. With these sparks he now charged the Leyden jar, and found, to his intense delight, that he could exhibit all the phenomena of electricity by means of the lightning he had drawn from the clouds.

In the following year a similar experiment, with even more striking results, was carried out, in France, by de Romas. Though it is said he had no knowledge of what Franklin had done in America, he, too, used a kite; and, with a view of making the string a better conductor, he interlaced with it a thin copper wire. Then, flying his kite in the ordinary way, when it had risen to a height of about 550 feet, he drew sparks from it which, we are told, were upwards of nine feet long, and emitted a sound like the report of a pistol.

Fatal Experiment of Richman.—There can be no doubt that experiments of this kind, made with the electricity of a thundercloud, were extremely dangerous; and this was soon proved by a fatal accident. Professor Richman, of St. Petersburgh, had erected on the roof of his house a pointed iron rod, the lower end of which passed into a glass vessel, intended, as we are informed, to measure the strength of the charge which he expected to receive from the clouds. On the sixth of August, 1753, observing the approach of a thunderstorm, he hastened to his apparatus; and as he stood near it, with his head bent down, to watch the effect, a flash of lightning passed through his body and killed him on the spot. This catastrophe served to fix public attention on the danger of such experiments, and gave occasion to the saying of Voltaire: “There are some great lords whom we should always approach with extreme precaution, and lightning is one of them.”[1] From this time the practice of making experiments directly with the lightning of the clouds seems to have been, by common consent, abandoned.

Immediate Cause of Lightning.—And now, having set before you some of the most memorable experiments by which the identity of lightning and electricity has been demonstrated, I will try to give you a clear conception regarding the immediate cause of lightning, so far as the subject is understood at the present day by scientific men. You know that there are two kinds of electricity, which are called positive and negative; and that each of them repels electricity of the same kind as itself, while it attracts electricity of the opposite kind. Now, every thundercloud is charged with electricity of one kind or the other, positive or negative; and, as it hovers over the earth, it develops, by what is called induction, or influence, electricity of the opposite kind in that part of the earth which is immediately under it. Thus we have two bodies—the cloud and the earth—charged with opposite kinds of electricity, and separated by a stratum of the atmosphere. The two opposite electricities powerfully attract each other; but for a time they are prevented from rushing together by the intervening stratum of air, which is a non-conductor of electricity, and acts as a barrier between them. As the electricity, however, continues to accumulate, the attraction becomes stronger and stronger, until at length it is able to overcome the resistance of this barrier; a violent disruptive discharge then takes place between the cloud and the earth, and the flash of lightning is the consequence of the discharge.

THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.

The whole phenomenon may be illustrated, on a small scale, by means of this electric machine of Carré’s which you see before you. When my assistant turns the handle of the machine negative electricity is developed in that large brass cylinder, which in our experiment will represent the thundercloud. At a distance of five or six inches from the cylinder I hold a brass ball, which is in electrical communication with the earth through my body. The electrified brass cylinder acts by induction, or influence on the brass ball, and develops in it, as well as in my body, a charge of positive electricity. Now, the positive electricity of the ball and the negative electricity of the cylinder are mutually attracting each other, but the intervening stratum of air offers a resistance which prevents a discharge from taking place. My assistant, however, continues to work the machine; the two opposite electricities rapidly accumulate on the cylinder and the ball; at length their mutual attraction is strong enough to overcome the resistance interposed between them; a disruptive discharge follows, and at the same moment a spark is seen to pass, accompanied by a sharp snapping report.

This spark is a miniature flash of lightning; and the snapping report is a diminutive peal of thunder. Furthermore, at the moment the spark passes you may observe a slight convulsive movement in my hand and wrist. This convulsive movement represents, on a small scale, the violent shock, generally fatal to life, which is produced by a flash of lightning when it passes through the body.