is exceedingly small, and any attempt to produce the electric currents required for lighting or other technical purposes by the use of a frictional machine is foredoomed to failure. Where currents of any magnitude are required, we must use other methods of producing electricity. These will be discussed subsequently, but for the present it is important to note that, apart from a question of degree, the frictional machine is an apparatus whereby electric currents may be generated.
How does the matter stand with regard to electrification by contact between solid bodies? Can we thereby also produce an electric current? We have seen that two metals in contact electrify each other. Using copper and zinc, the former becomes negatively and the latter positively electrified; that is to say, the zinc becomes the body of higher and the copper that of lower potential, and at first sight it might appear that by joining the back of the zinc disc to the back of the copper disc by a wire, we should get a current flowing along this wire from zinc to copper. This is, however, not the case.
Whatever the material of the joining wire may be, it must fall somewhere into the
