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.