68 ELEMENTARY LESSONS ON [CHAP. i. 



the potential of its charge rises as shown by the diverg- 

 ence of the gold leaves. To prove by the condensing 

 electroscope that contact of dissimilar metals does 

 produce electrification, a small compound bar made of 

 two dissimilar metals say zinc and copper soldered 

 together, is held in the hand, and one end of it is touched 

 against the lower plate, the upper plate being placed in 

 contact with the ground or touched with the finger. 

 When the two opposing charges have thus collected in 

 the condenser the upper plate is removed, and the 

 diverging of the gold leaves shows the presence of a 

 free charge, which can afterwards be examined to see 

 whether it be + or . For a long time the existence 

 of this electricity of contact was denied, or rather it was 

 declared to be due (when occurring in voltaic combina- 

 tions such as are described in Lesson XIII.) to chemical 

 actions going on ; whereas the real truth is that the 

 electricity of contact and the chemical action are both 

 due to molecular conditions of the substances which 

 come into contact with one another, though we do not 

 yet know the precise nature of the molecular conditions 

 which give rise to these two effects. Later experiments, 

 especially those made with the delicate electrometers of 

 Sir W. Thomson (Fig. 101), put beyond doubt the reality 

 of Volta's discovery. One simple experiment explains the 

 method adopted. A thin strip or 

 needle of metal is suspended so as 

 to turn about a point C. It is elec- 

 trified from a known source. Under 

 it are placed (Fig. 39) two semicir- 

 cular discs, or half-rings of dissimilar 

 metals. Neither attracts or repels 

 the electrified needle until the two are 

 brought into contact, or connected by 

 a third piece of metal, when the needle immediately turns, 

 being attracted by the one that is oppositely elecrified, and 

 repelled by the one that is similarly electrified with itself. 



