many other bodies which he called electrics, from the Greek word elektron, signifying amber. This great discovery was the starting-point of the modern science of electricity. That feeble and mysterious force which had been the wonder of the simple and the amusement of the vain could not be slighted any longer as a curious freak of nature, but assuredly none dreamt that a day was dawning in which it would transform the world.

Otto von Guericke, burgomaster of Magdeburg, was the first to invent a machine for exciting the electric power in larger quantities by simply turning a ball of sulphur between the bare hands. Improved by Sir Isaac Newton and others, who employed glass rubbed with silk, it created sparks several inches long. The ordinary frictional machine as now made is illustrated in figure i, where P is a disc of plate glass mounted on a spindle and turned by hand. Rubbers of silk R, smeared with an amalgam of mercury and tin, to increase their efficiency, press the rim of the plate between them as it revolves, and a brass conductor C, insulated on glass posts, is fitted with points like the teeth of a comb, which, as the electrified surface of the plate passes by, collect the electricity and charge the conductor with positive electricity. Machines of this sort have been made with plates 7 feet in diameter, and yielding sparks nearly 2 feet long.

The properties of the "electric fire," as it was now called, were chiefly investigated by Dufay. To refine on the primitive experiment let us replace the shreds by a pithball hung from a support by a silk thread, as in figure 2. If we rub the glass rod vigorously with a silk handkerchief and hold it near, the ball will fly toward the rod. Similarly we may rub a stick of sealing wax, a bar of sulphur, indeed, a great variety of substances, and by this easy test we shall find them electrified. Glass rubbed with glass will not show any sign of electrification, nor will wax rubbed on wax; but when the rubber is of a different material to the thing rubbed, we shall find, on using proper precautions, that electricity is developed. In fact, the property which was once thought peculiar to amber is found to belong to all bodies. ANY SUBSTANCE, WHEN RUBBED WITH A DIFFERENT SUBSTANCE, BECOMES ELECTRIFIED.

The electricity thus produced is termed frictional electricity. Of course there are some materials, such as amber, glass, and wax, which display the effect much better than others, and hence its original discovery.

In dry frosty weather the friction of a tortoise-shell comb will electrify the hair and make it cling to the teeth. Sometimes persons emit sparks in pulling off their flannels or silk stockings. The fur of a cat, or even of a garment, stroked in the dark with a warm dry hand will be seen to glow, and perhaps heard to crackle. During winter a person can electrify himself by shuffling in his slippers over the carpet, and light the gas with a spark from his finger. Glass and sealing-wax are, however, the most convenient means for investigating the electricity of friction.

A glass rod when rubbed with a silk handkerchief becomes, as we have seen, highly electric, and will attract a pithball (fig. 2). Moreover, if we substitute the handkerchief for the rod it will also attract the ball (fig. 3). Clearly, then, the handkerchief which rubbed the rod as well as the rod itself is electrified. At first we might suppose that the handkerchief had merely rubbed off some of the electricity from the rod, but a little investigation will soon show that is not the case. If we allow the pithball to touch the glass rod it will steal some of the electricity on the rod, and we shall now find the ball REPELLED by the rod, as illustrated in figure 4. Then, if we withdraw the rod and bring forward the handkerchief, we shall find the ball ATTRACTED by it. Evidently, therefore, the electricity of the handkerchief is of a different kind from that of the rod.

Again, if we allow the ball to touch the handkerchief and rub off some of its electricity, the ball will be REPELLED by the handkerchief and ATTRACTED by the rod. Thus we arrive at the conclusion that whereas the glass rod is charged with one kind of electricity, the handkerchief which rubbed it is charged with another kind, and, judging by their contrary effects on the charged ball or indicator, they are of opposite kinds. To distinguish the two sorts, one is called POSITIVE and the other NEGATIVE electricity.

Further experiments with other substances will show that sometimes the rod is negative while the rubber is positive. Thus, if we rub the glass rod with cat's fur instead of silk, we shall find the glass negative and the fur positive. Again, if we rub a stick of sealing-wax with the silk handkerchief, we shall find the wax negative and the silk positive. But in every case one is the opposite of the other, and moreover, an equal quantity of both sorts of electricity is developed, one kind on the rod and the other on the rubber. Hence we conclude that EQUAL AND OPPOSITE QUANTITIES OF ELECTRICITY ARE SIMULTANEOUSLY DEVELOPED BY FRICTION.

If any two of the following materials be rubbed together, that higher in the list becomes positively and the other negatively electrified:—

POSITIVE (+).

    Cats' fur.
    Polished glass.
    Wool.
    Cork, at ordinary temperature.
    Coarse brown paper.
    Cork, heated.
    White silk.
    Black silk.
    Shellac.
    Rough glass.

NEGATIVE (-).

The list shows that quality, as well as kind, of material affects the production of electricity. Thus polished glass when rubbed with silk is positive, whereas rough glass is negative. Cork at ordinary temperature is positive when rubbed with hot cork. Black silk is negative to white silk, and it has been observed that the best radiator and absorber of light and heat is the most negative. Black cloth, for instance, is a better radiator than white, hence in the Arctic regions, where the body is much warmer than the surrounding air, many wild animals get a white coat in winter, and in the tropics, where the sunshine is hotter than the body, the European dons a white suit.

The experiments of figures 1, 2, and 3 have also shown us that when the pithball is charged with the positive electricity of the glass rod it is REPELLED by the like charge upon the rod, and ATTRACTED by the negative or unlike charge on the handkerchief. Again, when it is charged with the negative electricity of the handkerchief it is REPELLED by the like charge on the handkerchief and ATTRACTED by the positive or unlike charge on the rod. Therefore it is usual to say that LIKE ELECTRICITIES REPEL AND UNLIKE ELECTRICITIES ATTRACT EACH OTHER.

We have said that all bodies yield electricity under the friction of dissimilar bodies; but this cannot be proved for every body by simply holding it in one hand and rubbing it with the excitor, as may be done in the case of glass. For instance, if we take a brass rod in the hand and apply the rubber vigorously, it will fail to attract the pithball, for there is no trace of electricity upon it. This is because the metal differs from the glass in another electrical property, and they must therefore be differently treated. Brass, in fact, is a conductor of electricity and glass is not. In other words, electricity is conducted or led away by brass, so that, as soon as it is generated by the friction, it flows through the hand and body of the experimenter, which are also conductors, and is lost in the ground. Glass on the other hand, is an INSULATOR, and the electricity remains on the surface of it. If, however, we attach a glass handle to the rod and hold it by that whilst rubbing it, the electricity cannot then escape to the earth, and the brass rod will attract the