No charge on the inside surface of a hollow charged conductor
The electric conductors used in the experiments we have described are generally made of hollow brass or else of wood covered with tinfoil. No advantage is to be gained by making them of solid metal, since the charge resides only on the outside surface. The following experiments illustrate this fact.
(I) The hol/ow charged conductor (Fig. 32.13 (a)) An insulated hollow brass sphere with a small hole in it is charged by induction,
and its inside and outside surfaces are then tested for charge by means of a proof
plane and electroscope. It is found that charge may readily be taken from the
outside, but none at all from the inside. (2) Faraday’s butterfly-net experiment (Fig. 32.13 (b)) M’ichael Faraday used an insulated cotton net to act as a hollow conductor. The net, to which an insulating silk string is attached, is given a charge by induction. Tests carried out with a proof plane and electroscope will now show that
charge may be taken from the outside of the net but not from the inside. The net is
next turned inside out by pulling the silk string and again tested for charge, The results are the same as before, showing that the charge must have passed from one side of the net to the other when it was turned inside out. (3) Coulomb’s hemispheres * (Fig. 32.14) Charles Coulomb demonstrated that charge always resides on the outside surface of a conductor with the aid of two hemispherical cups which fitted exactly round an insulated metal sphere.
The sphere is first charged, and afterwards the hemispheres are fitted over it while being held by insulating handles. On removing the hemispheres they are found to be charged, but no charge at all remains on the sphere. This shows that all the charge on the sphere must have passed to the outside of the hemispheres.