Although the science of electricity is of comparatively recent date, electricity itself has existed from the beginning of the world. There can be no doubt that man’s introduction to electricity was brought about through the medium of the thunderstorm, and from very early times come down to us records of the terror inspired by thunder and lightning, and of the ways in which the ancients tried to account for the phenomena. Even to-day, although we know what lightning is and how it is produced, a severe thunderstorm fills us with a certain amount of awe, if not fear; and we can understand what a terrifying experience it must have been to the ancients, who had none of our knowledge.
These early people had simple minds, and from our point of view they had little intelligence; but they possessed a great deal of curiosity. They were just as anxious to explain things as we are, and so they were not content until they had invented an explanation of lightning and thunder. Their favourite way of accounting for anything they did not understand was to make up a sort of romance about it. They believed that the heavens were inhabited by various gods, who showed their pleasure or anger by signs, and so they naturally concluded that thunder was the voice of angry gods, and lightning the weapon with which they struck down those who had displeased them. Prayers and sacrifices were therefore offered to the gods, in the hope of appeasing their wrath.
Greek and Roman mythology contains many references to thunder and lightning. For instance, we read about the great god Zeus, who wielded thunder-bolts which had been forged in underground furnaces by the giant Cyclops. There was no doubt that the thunder-bolts were made in this way, because one only had to visit a volcano in order to see the smoke from the furnace, and hear the rumbling echo of the far-off hammering. Then we are told the tragic story of Phaeton, son of the Sun-god. This youth, like many others since his time, was daring and venturesome, and imagined that he could do things quite as well as his father. On one occasion he tried to drive his father’s chariot, and, as might have been expected, it got beyond his control, and came dangerously near the Earth. The land was scorched, the oceans were dried up, and the whole Earth was threatened with utter destruction. In order to prevent such a frightful catastrophe, Jupiter, the mighty lord of the heavens, hurled a thunder-bolt at Phaeton, and struck him from the chariot into the river Po. A whole book could be written about these ancient legends concerning the thunderstorm, but, interesting as they are, they have no scientific value, and many centuries were to elapse before the real nature of lightning was understood.
In order to trace the first glimmerings of electrical knowledge we must leave the thunderstorm and pass on to more trivial matters. On certain sea-coasts the ancients found a transparent yellow substance capable of taking a high polish, and much to be desired as an ornament; and about 600 years B.C. it was discovered that this substance, when rubbed, gained the power of drawing to it bits of straw, feathers, and other light bodies. This discovery is generally credited to a Greek philosopher named Thales, 941–563 B.C., and it must be regarded as the first step towards the foundation of electrical science. The yellow substance was amber. We now know it to be simply a sort of fossilized resin, but the Greeks gave it a much more romantic origin. When Phaeton’s rashness brought him to an untimely end, his sorrowing sisters, the Heliades, were changed into poplar trees, and their tears into amber. Amongst the names given to the Sun-god was Alector, which means the shining one, and so the tears of the Heliades came to have the name Electron, or the shining thing. Unlike most of the old legends, this story of the fate of the Sun-maidens is of great importance to us, for from the word “electron” we get the name Electricity.
Thales and his contemporaries seem to have made no serious attempts to explain the attraction of the rubbed amber, and indeed so little importance was attached to the discovery that it was completely forgotten. About 321 B.C. one Theophrastus found that a certain mineral called “lyncurium” gained attractive powers when rubbed, but again little attention was paid to the matter, and astonishing as it may seem, no further progress worth mention was made until towards the close of the sixteenth century, when Doctor Gilbert of Colchester began to experiment seriously. This man was born about 1543, and took his degree of doctor of medicine at Cambridge in 1569. He was very successful in his medical work, and became President of the College of Physicians, and later on physician to Queen Elizabeth. He had a true instinct for scientific research, and was not content to accept statements on the authority of others, but tested everything for himself. He found that sulphur, resin, sealing-wax, and many other substances behaved like amber when rubbed, but he failed to get any results from certain other substances, such as the metals. He therefore called the former substances “electrics,” and the latter “anelectrics,” or non-electrics. His researches were continued by other investigators, and from him dates the science of electricity.
Fig. 1.—Suspended pith ball for showing electric attraction.
Leaving historical matters for the present, we will examine the curious power which is gained by substances as the result of rubbing. Amber is not always obtainable, and so we will use in its place a glass rod and a stick of sealing-wax. If the glass rod is rubbed briskly with a dry silk handkerchief, and then held close to a number of very small bits of paper, the bits are immediately drawn to the rod, and the same thing occurs if the stick of sealing-wax is substituted for the glass. This power of attraction is due to the presence of a small charge of electricity on the rubbed glass and sealing-wax, or in other words, the two substances are said to be electrified. Bits of paper are unsatisfactory for careful experimenting, and instead of them we will use the simple piece of apparatus shown in [Fig. 1]. This consists of a ball of elder pith, suspended from a glass support by means of a silk thread. If now we repeat our experiments with the electrified glass or sealing-wax we find that the little ball is attracted in the same way as the bits of paper. But if we look carefully we shall notice that attraction is not the only effect, for as soon as the ball touches the electrified body it is driven away or repelled. Now let us suspend, by means of a thread, a glass rod which has been electrified by rubbing it with silk, and bring near it in turn another silk-rubbed glass rod and a stick of sealing-wax rubbed with flannel. The two glass rods are found to repel one another, whereas the sealing-wax attracts the glass. If the experiment is repeated with a suspended stick of sealing-wax rubbed with flannel, the glass and the sealing-wax attract each other, but the two sticks of wax repel one another. Both glass and sealing-wax are electrified, as may be seen by bringing them near the pith ball, but there must be some difference between them as we get attraction in one case and repulsion in the other.
The explanation is that the electric charges on the silk-rubbed glass and on the flannel-rubbed sealing-wax are of different kinds, the former being called positive, and the latter negative. Bodies with similar charges, such as the two glass rods, repel one another; while bodies with unlike charges, such as the glass and the sealing-wax, attract each other. We can now see why the pith ball was first attracted and then repelled. To start with, the ball was not electrified, and was attracted when the rubbed glass or sealing-wax was brought near it. When however the ball touched the electrified body it received a share of the latter’s electricity, and as similar charges repel one another, the ball was driven away.
The kind of electricity produced depends not only on the substance rubbed, but also on the material used as the rubber. For instance, we can give glass a negative charge by rubbing it with flannel, and sealing-wax becomes positively charged when rubbed with silk. The important point to remember is that there are only two kinds of electricity, and that every substance electrified by rubbing is charged either positively, like the silk-rubbed glass, or negatively, like the flannel-rubbed sealing-wax.