Page:Electricity (1912) Kapp.djvu/39

produced by rubbing a glass rod with a pad of leather, which has been covered with a mercury amalgam of zinc. In this case the glass rod shows positive electrification, and the pad negative. The old physics textbooks, therefore, also speak of a "vitreous" and a "resinous" electricity, meaning thereby respectively electric charges of positive and negative sign. The electrification is the result of friction between two different substances, one becoming positively and the other negatively charged. Probably any pair of bodies can thus be electrified, provided the necessary care is taken to prevent the accumulated charge leaking off. The process is not even restricted to solids; the friction between a solid and a gas also produces electrification. This fact is utilised in the Armstrong electric machine, where jets of steam are caused to flow past the spikes of a metal comb. By the friction of the steam against the surface of the metal the latter becomes electrified. It is also well known that the friction of the gas escaping through the valve of a balloon produces electrification of the envelope, and under certain circumstances so strong an electrification that a spark discharge may occur. This danger is