120 ELEMENTS OF SCIENCE 



the disc on the other side to become negatively electrical 

 (and vice versa) without anything being directly done to 

 the opposite side. This, as it were spontaneous, evoca- 

 tion of a definite and antithetical kind of electricity, is 

 called electrical induction. 



When two bodies in opposite electrical conditions are 

 brought near each other they are mutually attracted, 

 and when they approach within a certain distance (the 

 amount of which varies with the intensity of the elec- 

 trical energy excited) the electrical energy will be mani- 

 fested by a flash of light accompanied by some heat and 

 a sound. Then occurs what, on a small scale, is called 

 an electric spark, and on a large scale is known as a 

 flash of lightning. When this has taken place sufficiently, 

 the opposite surfaces will be found to be no longer in 

 opposite electrical states; the " discharge," as it is called, 

 will have neutralised their opposition, and both will have 

 returned to their normal and unexcited condition. 



But a phenomenon much more conspicuous and familiar 

 than that of induction, is what is known as " conduction." 

 The pith balls and the glass disc were supposed to be 

 suspended by silk, because, while so suspended, they can 

 keep the electricity they acquire. If they were suspended 

 by a metal wire, they would not keep it at all, for it 

 would instantaneously run away through such a channel. 

 The metal wire takes, or " conducts" it away with 

 extreme rapidity and facility, metals being extremely 

 good conductors. Thus bodies may be arranged in two 

 opposite classes : " conductors " and " non-conductors." 

 Silk, glass, resin, wax, porcelain, and india-rubber are 

 all non-conductors, and woollen material, dry wood, and 

 leather conduct badly. Not only the metals, however, 

 but all objects containing much moisture, such as the 

 bodies of animals and plants, are good conductors. 



