Potential and the gold-leaf electroscope
In the previous chapter we looked upon a gold-leaf electroscope as an instrument chiefly for the purpose of indicating the presence or sign of an electric charge, or for making ‘a rough measurement of quantity of charge. Actually, the leaf divergence of
a gold-leaf electroscope indicates that there is a potential difference between the leaf and case.
Fig. 33.2 (a) illustrates the normal use of an electroscope to test for the presence of charge. The positive charge held above the cap raises the potential of the cap and leaf, while the potential of the case remains at zero, since it is earth-connected. The resultant difference of potential between leaf and case causes the leaf to diverge. In Fig. 33.2 (h) the electroscope has been placed upon a slab of paraffin wax so
that the case is insulated while the cap and leaf are earth-connected. If, say, a
negative charge is now brought near the case, the potential of the case is lowered while that of the leaf remains at zero. Once more a potential difference has been set up between leaf and case, and so the leaf diverges.
A third experiment is illustrated in Fig. 33.2 (c). This time the case is left standing on the wax slab and the cap and case are joined by a piece of copper wire. It is now impossible to cause the leaf to diverge, either by bringing near a charged rod or even by charging leaf and case by induction. Since leaf and case are connected together, they will always be at the same potential.