6. In charging an electroscope by induction, why must the finger be removed before the glass rod?
7. Why is it best to have the rubber and glass rods, used in electrification, warmer than the air of the room in which the experiments are being performed?
8. When a sharp metallic point is held near the knob of a charged electroscope the leaves quickly come together. Explain.
9. Might one of the members of your class in physics be charged with electricity, if he should stand on a board supported by dry glass insulators? Explain.
10. If a metal can is charged strongly while standing on an insulator, tests made by means of the proof-plane and electroscope show no charge on the inside. Explain.
(3) Electrical Theories and Distribution of Charges
222. Franklin's Theory of Electricity.—We have studied the production of electrification by friction and induction. It will be helpful now to consider some of the theories of electricity. From the ease with which electrification moves, along a conductor, many have imagined that electricity is a fluid. Benjamin Franklin's One Fluid Theory held that a positive charge consisted in an accumulation or an excess of electricity while a negative charge implies a deficiency or less than the usual amount. This theory led to representing positive electrification by a plus (+) sign and negative, by a minus (-) sign. These signs are in general use to-day. The use and significance of these signs should be clearly fixed in mind.
223. The Electron Theory.—Various discoveries and experiments made in recent years indicate, however, that negative electricity consists of little corpuscles or electrons which may pass readily from one molecule of a conductor to another while their movement through an insulator is much retarded if not entirely prevented. This theory, sometimes called the Electron Theory, holds that each atom of a substance has as a nucleus a corpuscle of positive electricity, and surrounding it, minute negative corpuscles or electrons. It is thought that the electrons in the atom are very much smaller than the positive charges and are revolving about the latter with great rapidity. Ordinarily, the positive and negative charges are equal so that the atom is in a neutral or uncharged condition. By the action of various forces some of the negative corpuscles within a conductor may be moved from molecule to molecule. Thus if a negatively charged rod is brought near a conductor, many electrons stream away to the far end charging it negatively, while the nearer end of the conductor is left with fewer electrons than usual along with the fixed positive corpuscles. Hence the near end is positively charged. (See Fig. 198.) On the other hand, if a positive charge is used, it attracts the electrons from the far end, leaving the immovable positive corpuscles there, and that end becomes positively electrified, while the nearer end with its surplus of electrons is, of course, negatively electrified.
The Electron Theory is considered well founded since the electrons have (a) had their mass determined, (b) their speed measured, (c) their electric charge determined, (d) and their behavior while passing through magnetic and electric fields observed. These facts and other experimental evidence have demonstrated the existence of electrons. The positive corpuscle has not been directly observed but is assumed to exist to account for the effects observed in induction, charging by friction, etc.