Page:Electricity (1912) Kapp.djvu/121

Rh This flow is impossible through the body of the glass, since glass is an insulator. Conversely, if we touch a charged glass sphere at one point we may take off a little of the charge, but not the whole of it, which is distributed over the sphere. Between electricity at rest and in motion, or as we may also say, between static charges and currents, there is thus the fundamental distinction that the static charge requires surface and the moving charge body.

The greater the current we wish to convey, the stouter must be the wire. The commercial unit of current is the Ampere, so named after the great French physicist. We may define the current existing in a particular point of the wire as the number of unit charges which pass that point during one second. The magnitude of the electrostatic unit of quantity has been defined in Chapter I, p. 13. By experiment it has been proved that one ampere is represented by the passage of 3000 millions of such units per second.

Man has no sense by which he can estimate the strength of a current flowing in a wire—by touching the wire he may get a shock and thus conclude that the wire is at a higher potential than he is himself when standing on the earth; and indeed wiremen, who have