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84 that this is also the potential right through the cavity of the hollow sphere. This law that the potential at any point inside a conductor is the same as the potential on its surface, may also be proved without the aid of mathematics by the following reasoning: Imagine a conductor of any shape, and assume it at first to be solid right through. No free electricity could possibly remain in the substance of the metal, since the mutual repulsion of all the elementary charges would cause these to try to move apart as far as they can. As the carrier of these charges is metallic, that is to say, offers no resistance to the free displacement or flow of electricity, there is nothing to hinder the movement, and consequently the charge will all accumulate on the outside surface. There is, therefore, no charge in the body of the metal, and we may, without changing the electrical condition, take away the inside and leave only the merest shell, and still there can be no electrical effect produced inside the shell. We may charge such a shell with the strongest machine made, and yet in the inside not a trace of electricity can be detected.

This has first been proved experimentally by Faraday, who constructed for the purpose