The parallel-plate capacitor
A flat metal plate, A, is set up vertically on insulating legs and is connected to a goldleaf electroscope by means of a wire. The plate is then given a positive charge by induction with a negatively charged ebonite’rod. The divergence of the leaf indicates
the potential of the plate (Fig. 33.9 (a».
A second insulated plate B is now brought up slowly into a position parallel to A. When B is very close to A but not touching it, it will be noticed that the leaf divergence decreases very slightly. We conclude from this that the potential of A has been decreased by the presence of B, and hence its capacitance has increased slightly (Fig. 33.9 (b».
This has been brought about as follows. The positive charge on A induces equal and opposite charges on opposite sides of B. These induced charges will respectively raise and lower the potential of all points in their neighbourhood and, in particular, they will affect the potential of plate A. As far as A is concerned, however, the negative induced charge will have the greater effect, since it is closer to A than the positive charge. The net result is that the potential of A is slightly reduced. B is next earthed either by touching it with the finger or by connecting it to the nearest cold-water pipe (Fig. 33.9 (e». Immediately the leaf shows a big decrease in divergence. This implies a big decrease in potential, and hence a big increase in the capacitance of A.
When B was earthed its positive induced charge disappeared. Previously this charge was helping to raise the potential of A. Now, only the negative induced charge remains and acts alone to cause a big decrease in the potential of A.