If the Leyden jars of a Wimshurst machine are connected up and the discharging balls placed at a suitable distance apart, the electricity produced by rotating the plates is discharged in the form of a brilliant zigzag spark between the balls, accompanied by a sharp crack. The resemblance between this spark and forked lightning is at once evident, and in fact it is lightning in miniature. The discharging balls are charged, as we have seen, with opposite kinds of electricity, and these charges are constantly trying to reach one another across the intervening air, which, being an insulator, vigorously opposes their passage. There is thus a kind of struggle going on between the air and the two charges of electricity, and this keeps the air in a state of constant strain. But the resisting power of the air is limited, and when the charges reach a certain strength the electricity violently forces its way across, literally rupturing or splitting the air. The particles of air along the path of the discharge are rendered incandescent by the heat produced by the passage of the electricity, and so the brilliant flash is produced. Just as a river winds about seeking the easiest course, so the electricity takes the path of least resistance, which probably is determined by the particles of dust in the air, and also by the density of the air, which becomes compressed in front, leaving less dense air and therefore an easier path on each side.
The connexion between lightning and the sparks from electrified bodies and electrical machines was suspected by many early observers, but it remained for Benjamin Franklin to prove that lightning was simply a tremendous electric discharge, by actually obtaining electricity from a thunder-cloud. Franklin was an American, born at Boston in 1706. He was a remarkable man in every way, and quite apart from his investigations in electricity, will always be remembered for the great public services he rendered to his country in general and to Philadelphia in particular. He founded the Philadelphia Library, the American Philosophical Society, and the University of Pennsylvania.
Franklin noticed many similarities between electricity and lightning. For instance, both produced zigzag sparks, both were conducted by metals, both set fire to inflammable materials, and both were capable of killing animals. These resemblances appeared to him so striking that he was convinced that the two were the same, and he resolved to put the matter to the test. For this purpose he hit upon the idea of using a kite, to the top of which was fixed a pointed wire. At the lower end of the flying string was tied a key, insulated by a piece of silk ribbon. In June 1752, Franklin flew his kite, and after waiting a while he was rewarded by finding that when he brought his knuckle near to the key a little spark made its appearance. This spark was exactly like the sparks obtained from electrified bodies, but to make things quite certain a Leyden jar was charged from the key. Various experiments were then performed with the jar, and it was proved beyond all doubt that lightning and electricity were one and the same.
Lightning is then an enormous electric spark between a cloud and the Earth, or between two clouds, produced when opposite charges become so strong that they are able to break down the intervening non-conducting layer of air. The surface of the Earth is negatively electrified, the electrification varying at different times and places; while the electricity of the air is usually positive, but frequently changes to negative in rainy weather and on other occasions. As the clouds float about they collect the electricity from the air, and thus they may be either positively or negatively electrified, so that a discharge may take place between one cloud and another, as well as between a cloud and the Earth.
Lightning flashes take different forms, the commonest being forked or zigzag lightning, and sheet lightning. The zigzag form is due to the discharge taking the easiest path, as in the case of the spark from a Wimshurst machine. Sheet lightning is probably the reflection of a flash taking place at a distance. It may be unaccompanied by thunder, as in the so-called “summer lightning,” seen on the horizon at night, which is the reflection of a storm too far off for the thunder to be heard. A much rarer form is globular or ball lightning, in which the discharge takes the shape of a ball of light, which moves slowly along and finally disappears with a sudden explosion. The cause of this form of lightning is not yet understood, but it is possible that the ball of light consists of intensely heated and extremely minute fragments of ordinary matter, torn off by the violence of the lightning discharge. Another uncommon form is multiple lightning, which consists of a number of separate parallel discharges having the appearance of a ribbon.
A lightning flash probably lasts from about 1/100,000 to 1/1,000,000 of a second, and in the majority of cases the discharge is oscillatory; that is to say, it passes several times backwards and forwards between two clouds or between a cloud and the Earth. At times it appears as though we could see the lightning start downwards from the cloud or upwards from the Earth, but this is an optical illusion, and it is really quite impossible to tell at which end the flash starts.
Death by lightning is instantaneous, and therefore quite painless. We are apt to think that pain is felt at the moment when a wound is inflicted. This is not the case however, for no pain is felt until the impression reaches the brain by way of the nerves, and this takes an appreciable time. The nerves transmit sensations at a speed of only about one hundred feet per second, so that in the case of a man killed by a bullet through the brain, no pain would be felt, because the brain would be deprived of sensibility before the sensation could reach it. Lightning is infinitely swifter than any bullet, so life would be destroyed by it before any pain could be felt.
On one occasion Professor Tyndall, the famous physicist, received accidentally a very severe shock from a large battery of Leyden jars while giving a public lecture. His account of his sensations is very interesting. “Life was absolutely blotted out for a very sensible interval, without a trace of pain. In a second or so consciousness returned; I saw myself in the presence of the audience and apparatus, and, by the help of these external appearances, immediately concluded that I had received the battery discharge. The intellectual consciousness of my position was restored with exceeding rapidity, but not so the optical consciousness. To prevent the audience from being alarmed, I observed that it had often been my desire to receive accidentally such a shock, and that my wish had at length been fulfilled. But, while making this remark, the appearance which my body presented to myself was that of a number of separate pieces. The arms, for example, were detached from the trunk, and seemed suspended in the air. In fact, memory and the power of reasoning appeared to be complete long before the optic nerve was restored to healthy action. But what I wish chiefly to dwell upon here is, the absolute painlessness of the shock; and there cannot be a doubt that, to a person struck dead by lightning, the passage from life to death occurs without consciousness being in the least degree implicated. It is an abrupt stoppage of sensation, unaccompanied by a pang.”
Occasionally branched markings are found on the bodies of those struck by lightning, and these are often taken to be photographic impressions of trees under which the persons may have been standing at the time of the flash. The markings however are nothing of the kind, but are merely physiological effects due to the passage of the discharge.
During a thunderstorm it is safer to be in the house than out in the open. It is probable that draughts are a source of some danger, and the windows and doors of the room ought to be shut. Animals are more liable to be struck by lightning than men, and a shed containing horses or cows is a dangerous place in which to take shelter; in fact it is better to remain in the open. If one is caught in a storm while out of reach of a house or other building free from draughts and containing no animals, the safest plan is to lie down, not minding the rain. Umbrellas are distinctly dangerous, and never should be used during a storm. Wire fences, hedges, and still or running water should be given a wide berth, and it is safer to be alone than in company with a crowd of people. It is extremely foolish to take shelter under an isolated tree, for such trees are very liable to be struck. Isolated beech trees appear to have considerable immunity from lightning, but any tree standing alone should be avoided, the oak being particularly dangerous. On the other hand, a fairly thick wood is comparatively safe, and failing a house, should be chosen before all other places of refuge. Horses are liable to be struck, and if a storm comes on while one is out driving it is safer to keep quite clear of the animals.