Experiments with a gold-leaf electroscope
(1) To detect the presence of charge on a body
If a rod of some suitable material is charged by friction and then brought near to the cap of a gold-leaf electroscope the leaf is seen to diverge from the plate. A charge has been induced on the leaf and plate, and consequently repulsion occurs between them. On removing the charged rod, the leaf collapses, showing that the induced charge on the electroscope is only temporary. Very small charges may be detected by this method.
(2) To charge a gold-leaf electroscope by contact
Generally speaking, it is not always easy to charge an electroscope by contact with a charged rod, but usually it can be done after a fe~ttempts. An ebonite rod is given a small charge by rubbing with fur, and is then rolled over the cap of an electroscope. The leaf will be seen to diverge, and then the rod is removed. If the leaf does not stay diverged the process is repeated until it does. We
may now assume that the electroscope is charged with negative electricity by conduction from the ebonite rod. If the cap of the electroscope is touched with the finger the charge flows to earth through the experimenter’s body and the leaf collapses. This is called “earthing the electroscope”. Before proceeding any further it must be pointed out that charging by contact is not a good method and often gives a charge opposite to that expected. It is better to use the method of induction, which is described later.
(3) To test for the sign of the charge on a body
Having charged the electroscope negatively as described above, the ebonite rod should be recharged and brought near to the cap. An increase in the leaf divergence is noted. A glass rod rubbed with silk (positive charge) is now cautiously brought down
towards the cap from a height of about 50 em. This time, a decrease in divergence is noticed. The electroscope is discharged by touching it with the finger and afterwards charged positively by contact, using a glass rod rubbed with silk. We shall now find that an increased divergence is caused by bringing a charged glass rod near the cap and a decreased divergence by a charged ebonite rod. From these experiments we conclude that an increase in divergence occurs when the charge on the electroscope and the test charge are of the same kind. We may not assume, however, that.a decrease in divergence necessarily means that a charge of opposite kind has been brought near a charged electroscope. An uncharged body has the same effect. One’s own hand is a convenient uncharged body to use for this purpose. 11follows that the only sure test for the sign of charge on a body is to obtain an increase in divergence. The results of these experiments are summarized in the table shown.
(4) To test the insulating properties of various materials
The insulating or, conversely, the conducting property of a given substance may be tested by holding a sample of the substance in the hand and then bringing it into contact with the cap of a charged electroscope. If the substance is a good insulator there will be no leakage of charge through it and the leaf divergence will not alter. If, however, the leaf collapses instantly it shows that the substance is a good conductor.
Between these two extremes there are certain substances which produce a slow collapse of the leaf. These are classed as poor insulators or poor conductors. Examples of t his type of material are paper, wool, cotton, and wood. Nevertheless, if these substances are dried thoroughly they become quite good insulators. This suggests that their ability to conduct electricity comes from their moisture content. Among the good conductors are all the metals and carbon. The good insulators are sulphur, quartz, paraffin wax, polyvinyl chloride (P.V.c.), shellac, polythene, and silk. The experiment should be carried out with as many different substances as possible and the results recorded in a table as below.