The proof plane
When testing a charge on a conductor it is often necessary to transfer a small sample of the charge to a gold-leaf electroscope. This is done by means of a proof plane, which consists of a small metal disc at the end of an insulating handle (Fig. 32.3).The proof plane is placed in contact with the surface of the conductor so that it becomes charged, and the charge is then transferred to the electroscope by touching the cap with the proof plane. Fig. 32.4 illustrates the use of a proof plane to show the existence of ind uced charges on a conductor AB when a positively charged glass rod is brought near it. The proof plane is first of all used to transfer some of the charge from the end B of the conductor to the electroscope. The sign of this charge is then tested by bringing a
positively charged rod slowly down towards t he cap of the electroscope from a goodheight above it. An increase in divergence of the leaf will show that the charge taken from B is positive.
The electroscope and the proof plane are now both discharged by touching them with the finger and the experiment repeated using charge taken from the end A of the conductor. This time an increase in divergence is obtained when a negatively charged ebonite rod is brought near the cap, showing that the charge taken from A is negative.
The reason why the charged rods must be brought down towards the cap of the
electroscope from a good height above it is explained on page 372.