5. Are the rods upon which we produce electrification by friction, conductors or insulators? How do you explain this?
6. Are conductors or insulators of the greater importance in practical electricity? Explain.
(2) Electric Fields and Electrostatic Induction
Fig. 192.—An electric field about a positively charged shell.
Fig. 193.—A "detector."
218. Electrical Fields.—In our study of magnetism we learned that a magnet affects objects about it by its magnetic lines of force. In a similar way it is assumed that a charged body produces electrical effects upon its surroundings by electric lines of force. For example, the attraction that a charged body exerts upon light objects through short distances or the influence of a charge upon an electroscope several feet away, is said to be due to the electric field about the charged body. (See Fig. 192.) The presence of the electric lines of force may be shown by placing a perforated, slender, diamond-shaped piece of tissue paper upon a light glass pointer (Fig. 193). When placed in an electric field the tissue paper "detector" places itself parallel to the lines of force. Electric lines of force are said to extend from a positive to a negative charge. (See Fig. 194.) The direction shown by the arrow upon the lines is that along which a small positive charge tends to move. Electric lines of force unlike those from magnets are not continuous. They extend from a positive charge to a negative charge. Therefore each positive charge is connected by lines of force to a negative charge somewhere. These ideas of electric fields are of much assistance in explaining many electrical effects. Electrical fields between oppositely charged shells will be found similar to Fig. 194, while between shells with like charges, fields are found as in Fig. 195.
Fig. 194.—Electric field between unlike charges.
Fig. 195.—Electric field between like charges.