When a Wimshurst machine has been in action for a little time a peculiar odour is noticed. This is due to the formation of a modified and chemically more active form of oxygen, called ozone, the name being derived from the Greek ozein, “to smell.” Ozone has very invigorating effects when breathed, and it is also a powerful germicide, capable of killing the germs which give rise to contagious diseases. During a thunderstorm ozone is produced in large quantities by the electric discharges, and thus the air receives as it were a new lease of life, and we feel the refreshing effects when the storm is over. We shall speak again of ozone in [Chapter XXV].
Thunder probably is caused by the heating and sudden expansion of the air in the path of the discharge, which creates a partial vacuum into which the surrounding air rushes violently. Light travels at the rate of 186,000 miles per second, and therefore the flash reaches us practically instantaneously; but sound travels at the rate of only about 1115 feet per second, so that the thunder takes an appreciable time to reach us, and the farther away the discharge the greater the interval between the flash and the thunder. Thus by multiplying the number of seconds which elapse between the flash and the thunder by 1115, we may calculate roughly the distance in feet of the discharge. A lightning flash may be several miles in length, the greatest recorded length being about ten miles. The sounds produced at different points along its path reach us at different times, producing the familiar sharp rattle, and the following rolling and rumbling is produced by the echoes from other clouds. The noise of a thunder-clap is so tremendous that it seems as though the sound would be heard far and wide, but the greatest distance at which thunder has been heard is about fifteen miles. In this respect it is interesting to compare the loudest thunder-clap we ever heard with the noise of the famous eruption of Krakatoa, in 1883, which was heard at the enormous distance of nearly three thousand miles.
When Franklin had demonstrated the nature of lightning, he began to consider the possibility of protecting buildings from the disastrous effects of the lightning stroke. At that time the amount of damage caused by lightning was very great. Cathedrals, churches, public buildings, and in fact all tall edifices were in danger every time a severe thunderstorm took place in their neighbourhood, for there was absolutely nothing to prevent their destruction if the lightning chanced to strike them. Ships at sea, too, were damaged very frequently by lightning, and often some of the crew were killed or disabled. To-day, thanks to the lightning conductor, it is an unusual occurrence for ships or large buildings to be damaged by lightning. The lightning strikes them as before, but in the great majority of cases it is led away harmlessly to earth.
Franklin was the first to suggest the possibility of protecting buildings by means of a rod of some conducting material terminating in a point at the highest part of the building, and leading down, outside the building, into the earth. Lightning conductors at the present day are similar to Franklin’s rod, but many improvements have been made from time to time as our knowledge of the nature and action of the lightning discharge has increased. A modern lightning conductor generally consists of one or more pointed rods fixed to the highest parts of the building, and connected to a cable running directly to earth. This cable is kept as straight as possible, because turns and bends offer a very high resistance to the rapidly oscillating discharge; and it is connected to large copper plates buried in permanently moist ground or in water, or to water or gas mains. Copper is generally used for the cable, but iron also may be employed. In any case, the cable must be of sufficient thickness to prevent the possibility of its being deflagrated by the discharge. In ships the arrangements are similar, except that the cable is connected to the copper sheathing of the bottom.
The fixing of lightning conductors must be carried out with great care, for an improperly fixed conductor is not only useless, but may be a source of actual danger. Lightning flashes vary greatly in character, and while a carefully erected lightning conductor is capable of dealing with most of them, there are unfortunately certain kinds of discharge with which it now and then is unable to deal. The only absolutely certain way of protecting a building is to surround it completely by a sort of cage of metal, but except for buildings in which explosives are stored this plan is usually impracticable.
The electricity of the atmosphere manifests itself in other forms beside the lightning. The most remarkable of these manifestations is the beautiful phenomenon known in the Northern Hemisphere as the Aurora Borealis, and in the Southern Hemisphere as the Aurora Australis. Aurora means the morning hour or dawn, and the phenomenon is so called from its resemblance to the dawn of day. The aurora is seen in its full glory only in high latitudes, and it is quite unknown at the equator. It assumes various forms, sometimes appearing as an arch of light with rapidly moving streamers of different colours, and sometimes taking the form of a luminous curtain extending across the sky. The light of the aurora is never very strong, and as a rule stars can be seen through it. Auroras are sometimes accompanied by rustling or crackling sounds, but the sounds are always extremely faint. Some authorities assert that these sounds do not exist, and that they are the result of imagination, but other equally reliable observers have heard the sounds quite plainly on several occasions. Probably the explanation of this confliction of evidence is that the great majority of auroras are silent, so that an observer might witness many of them without hearing any sounds. The height at which auroras occur is a disputed point, and one which it is difficult to determine accurately; but most observers agree that it is generally from 60 to 125 miles above the Earth’s surface.
There is little doubt that the aurora is caused by the passage of electric discharges through the higher regions of the atmosphere, where the air is so rarefied as to act as a partial conductor; and its effects can be imitated in some degree by passing powerful discharges through tubes from which the air has been exhausted to a partial vacuum. Auroral displays are usually accompanied by magnetic disturbances, which sometimes completely upset telegraphic communication. Auroras and magnetic storms appear to be connected in some way with solar disturbances, for they are frequently simultaneous with an unusual number of sunspots, and all three run in cycles of about eleven and a half years.
CHAPTER IV
THE ELECTRIC CURRENT
In the previous chapters we have dealt with electricity in charged bodies, or static electricity, and now we must turn to electricity in motion, or current electricity. In [Chapter I]. we saw that if a metal rod is held in the hand and rubbed, electricity is produced, but it immediately escapes along the rod to the hand, and so to the earth. In other words, the electricity flows away along the conducting path provided by the rod and the hand. When we see the word “flow” we at once think of a fluid of some kind, and we often hear people speak of the “electric fluid.” Now, whatever electricity may be it certainly is not a fluid, and we use the word “flow” in connexion with electricity simply because it is the most convenient word we can find for the purpose. Just in the same way we might say that when we hold a poker with its point in the fire, heat flows along it towards our hand, although we know quite well that heat is not a fluid. In the experiment with the metal rod referred to above, the electricity flows away instantly, leaving the rod unelectrified; but if we arrange matters so that the electricity is renewed as fast as it flows away, then we get a continuous flow, or current.
Somewhere about the year 1780 an Italian anatomist, Luigi Galvani, was studying the effects of electricity upon animal organisms, using for the purpose the legs of freshly killed frogs. In the course of his experiments he happened to hang against an iron window rail a bundle of frogs’ legs fastened together with a piece of copper wire, and he noticed that the legs began to twitch in a peculiar manner. He knew that a frog’s leg would twitch when electricity was applied to it, and he concluded that the twitchings in this case were caused in the same way. So far he was quite right, but then came the problem of how any electricity could be produced in these circumstances, and here he went astray. It never occurred to him that the source of the electricity might be found in something quite apart from the legs, and so he came to the conclusion that the phenomenon was due to electricity produced in some mysterious way in the tissues of the animal itself. He therefore announced that he had discovered the existence of a kind of animal electricity, and it was left for his fellow-countryman, Alessandro Volta, to prove that the twitchings were due to electricity produced by the contact of the two metals, the iron of the window rail and the copper wire.