Page:Electricity (1912) Kapp.djvu/46

42 The tendency will be to bring the conductors together, and if they are held firmly in place, the tendency will be for the charges themselves to leave the conductors and unite. Whether they will actually do this depends on the distance between the nearest points of the conductors and the strength of the charges accumulated on them. Under certain conditions the force of attraction may be sufficiently great and the distance sufficiently small to cause electricity to leap across the intervening space, and then we have the familiar phenomenon of an electric spark.

The same phenomenon is observed in lightning, in which case the conductors may be two clouds charged with electricity, or a cloud and the earth. The force which in an electric machine causes the separation between positive and negative electricity is called the "electromotive force," and the practical unit in which the magnitude of electromotive force is expressed is called the "volt." To give the reader an idea of the size of this unit it may be mentioned that the electromotive force (or e.m.f.) with which electricity is caused to flow through an incandescent lamp is of the order of 100 to