ELECTRICAL INDUCTION. 563 



the electrified body must be brought as near as possible, 

 yet without losing electricity by transmitting it to the 

 two metal discs, it is necessary to ascertain by pre- 

 liminary experiments how near to the discs the glass rod 

 may be brought without direct transmission of elec- 

 tricity. Care being taken that the two discs touch one 

 another while the glass rod is near them, the glass rod 



FIG. 304 (an.proj. real size). 



is removed, and each of the discs tested by bringing it 

 near to the electric pendulum : neither of them should 

 manifest electricity. By several successive experiments 

 the least distance between discs and glass rod is thus 

 determined at which no transmission of electricity takes 

 place. 



The excited glass rod is now again brought near to 

 the two coins, which are touching one another, but this 

 time the disc a in fig. 304 is immediately after the 

 approach of the glass rod removed to a distance of a 

 few millimetres from the disc 6, and the glass rod itself 

 is then put away altogether. Both discs are now suc- 

 cessively brought near to the electrical pendulum, . 

 which has a conducting suspension (cotton thread); 

 loth will be found to be charged equally strongly. In 

 order to ascertain the kind of electricity with which 



o o 2 



