250 volts, according to the type of lamp used. The most prevalent voltage employed for domestic lighting is 220 v. In comparison with this the e.m.f. of contact is very small, and that produced in a frictional machine is prodigiously large. The latter may easily reach tens of thousands or even hundreds of thousands of volts, whilst the e.m.f., under which lightning flashes occur, may be many millions of volts. Between the process of producing electrification by friction and producing it by contact, there is thus an enormous difference in degree, but no difference in kind, both processes being simply directed to the separation and isolation of charges of different sign.
If the positive and negative conductors of a frictional machine are connected by a wire, the charges rush along this wire to equalise each other, leaving both conductors without charge. We may imagine a simultaneous movement of positive and negative electricity along this wire in opposite directions, or we may imagine only the positive charge flowing along the wire in the direction of the negative conductor and spreading itself over its surface, and thereby neutralising the negative charge previously accumulated on it. What pre-
