2 STATIC ELECTRICITY 



Thus, resinous materials, such as sealing-wax or vulcanite, should 

 be rubbed with wool or fur, and glass with silk. 



Actions between bodies both electrified by friction. 

 We may study electric actions between bodies in more detail by 

 means of some such apparatus as the following, which we may term a 

 disc electroscope. Two discs, , b (Fig. 1), are attached to the ends 

 of a vulcanite or varnished glass rod, pivoted and free to turn on 

 an upright support ; a is glass and b is vulcanite ; c, d are two other 

 discs, of glass and vulcanite respectively, provided with vulcanite 

 handles. On rubbing a and c with silk, and b and d with wool or 

 fur, all the surfaces are electrified. On presenting the glass disc c 

 to the glass disc a, we find that it repels it. We find also that the 

 vulcanite disc d repels the disc i, or that the similar bodies repel 



FIG. 1. 



each other when electrified. On the other hand, c attracts l>, and 

 d attracts a. 



Two kinds of electrification : vitreous or positive, 

 resinous or negative. We have, then, two distinct kinds of 

 electrification that of the rubbed glass and that of the rubbed 

 vulcanite. Further experiments show that there arc only t! 

 two kinds, and they are often termed, from their simplest modes of 

 production, vitreous and resinous respectively. But it is to he 

 observed that when acting on a given electrified body they exert 

 opposite forces, which tend to neutralise each other. Thus the 

 detached discs c and d may be so electrified that if held close 

 together in front of a or of b they will produce no motion, their 

 two actions being equal and opposite. 



Hence, in regard to their mechanical actions, we may apply to 

 the two electrifications the terms positive and negative. Vitreous 

 or glass electrification is always taken as positive, and therefore 

 resinous electrification is negative. 



We may now make the following statement : 



Law of electric mechanical action. Bodies if similarly 

 electrified repel each other, and if oppositely electrified attract each 

 other. 



Two bodies rubbed together are oppositely 

 electrified. This may easily be proved by means of the sus- 

 pended glass and vulcanite discs (Fig. 1), electrifying these by 



