3. Should the metal top of an electroscope have sharp corners? Explain.
4. Would a tall steel tower have the same need of a lightning rod as a brick chimney of the same height? Explain.
5. Will a solid sphere hold a greater charge of electricity than a hollow one of the same diameter? Explain.
6. If a positively charged cloud floats over a tree which is a good conductor of electricity will the tree be charged? Show diagram. Explain.
(4) Potential, Capacity and the Electric Condenser
227. Conditions Causing a Movement of Electricity.—In the study of conductors and insulators it was observed that an electric charge moved along the conducting rod to the electroscope. This movement of electricity along a conductor is a result of great practical importance. We will now consider the conditions that produce the "flow" or "current" of electricity. Let two electroscopes stand near each other. Charge one, C´ (Fig. 206), strongly and charge the other slightly. If now a light stiff wire attached to a stick of sealing wax be placed so as to connect the tops of the electroscopes, the leaves of C will partly close while those of D will open slightly, thus indicating a movement of electricity from C to D along the wire. The movement was from a place of greater degree of electrification to one of less.
Fig. 206.—Electricity flows from high to low potential.
228. Potential.—The potential of an electrified body is its degree of electrification. Therefore, it is said that electroscope C mentioned above has a greater potential than electroscope D. The movement of electricity is from a place of greater or high potential to one of lesser or low potential. If two bodies are at the same potential there will be found no movement of electricity between them. A difference of potential between two points connected by a conductor is therefore the necessary condition for an electric current. Just as heat is transmitted along a conductor from a place of high to one of lower temperature, so electricity is transmitted along a conductor from a place of high to one of low potential. Thus potential in electricity corresponds to temperature in heat. One is the "degree of electrification," the other, "the degree of hotness."