CONDUCTION OF ELECTRICITY. 557 



placed upon the table, these will first be attracted, but 

 as soon as they touch the rod they are as strongly re- 

 pelled. They have, by contact with the glass rod, be- 

 come electric, and as their electricity is the same as that 

 of the rod, they are repelled by it. 



The same fact may be shown by the electrical pen- 

 dulum, provided that the ball is suspended by a thread 

 of silk, not one of cotton or linen for reasons which 

 will soon become apparent. The suspended ball is first 

 attracted, and after contact repelled, by the glass rod. 

 That the ball is now electric is shown by bringing the 

 hand near to it; it is attracted by the hand. By contact 

 with the hand, however, the ball loses its electricity. 



The silk thread must bo very fine ; a single fibre wound off a 

 cocoon is best for the purpose. If this cannot be obtained, a fibre as 

 fine and long as possible should be carefully drawn out of a thread 

 of untwisted (floss) silk. The length of the pendulum should not be 

 less than 15 cm . Such a fine fibre cannot be well prevented by a knot 

 from slipping through the hole made by the needle ; it should 

 therefore be attached to the ball with a very small bit of wax which 

 has been softened between the fingers. 



The silk thread must be dried, if the experiments are to succeed. 

 Over the lamp the thread is easily scorched ; it is best to hold it 

 near a fire, or to wrap it round a wire which has been made as hot 

 as will allow of its still being handled without inconvenience. 



A rod of glass or sealing-wax which has been rubbed 

 only at one end becomes electric only at the end at 

 which it has been rubbed; only this end attracts the 

 pendulum on the silk thread, and repels it after contact. 

 Many other bodies which have been electrified by con- 

 tact behave differently. A pretty long brass wire, about 

 pim thi c ]^ hag one en( } turned into a small loop, c in fig. 

 303 A ; a similar loop is made at the other end, and the 

 wire bent downwards ; to the extremity a flat disc of 



