=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. [Illustration: FIG. 194.--Electric field between unlike charges.] [Illustration: FIG. 195.--Electric field between like charges.] =219. Electrostatic Induction.=--If a charged body is brought near an aluminum-foil electroscope, the leaves separate. (See Fig. 198.) The nearer the charge is brought the wider the leaves spread, but when the charge is removed, the leaves collapse showing that nothing was given to the electroscope. It was simply affected by the charge in its vicinity. _This production of an electrified condition in a body by the influence of a charge near it is called electrostatic induction._ Placing insulators, such as a sheet of glass, between the charge and the electroscope does not affect the result, which is apparently brought about by the action of the electric lines of force. These lines of force extend without difficulty _through uncharged insulators_ and terminate often at the surface of a conductor, where their influence causes a charge to accumulate. _Charged insulators_, however, do affect inductive action. This may be noticed by using a sensitive electroscope. [Illustration: FIG. 196.--Production of two charges by the influence of a third charge.] [Illustration: FIG. 197.--The two charges separated.]