Page:Electricity (1912) Kapp.djvu/25

Rh force between two lead spheres of known mass, and has thus determined $$f$$, and from this value he found the mass of the earth to be $$6.6 \times 10^{27}$$ grams. In popular language, he has weighed the earth.

The reader may perhaps ask what all this has to do with electricity. Nothing directly. I have merely introduced the subject of gravitation, which is familiar to all, as a starting-point, so as to familiarise the reader with the conception of the ethereal coefficient; and I now go back to the consideration of electric and magnetic forces acting across space.

I assume that the reader is familiar with the usual textbook explanation of how bodies may be electrified, or, as it is also termed, charged with electricity. Imagine then that we have given electric charges to two spheres which are suspended from silk threads. Such suspension is necessary, for if we were to handle the spheres or lay them on to the table their charges would leak away; if we wish a body to preserve its charge for a sensible time we must support it by an insulator—such as silk, glass, mica, ebonite, which does not allow electricity to flow along or through it. Metals offer a very easy path for the flow of electricity, and are therefore called conductors.