TESTS OF ELECTRIC EXCITATION. 145 



three of them. If the balls B c, excited by con- 

 tact with a piece of rubbed amber, A, or sealing- 

 wax, Fig. 57, and while repelled therefrom after 

 contact, be brought near a piece of rubbed glass, 

 they are attracted thereby, because the vitreous 

 currents circulate in opposite directions to those 

 about the amber. 



Fig- 57- 



The reverse direction of the current about the 

 glass (which takes the place of A) coincides with 

 the currents about B and c, with the result of 

 reciprocal attraction between the three. 



In tracing out the movements of the balls of 

 electroscopes toward and from each other, as tests 

 of electric excitation, it is to be remembered that 

 the original mechanical causes of their movements 

 are the axial and orbital revolutions of the planets, 

 which combine to produce the various resultant 

 motions of electrodes, as has been previously 

 illustrated, and the revolving movements of 

 various mechanisms by the reciprocal actions 

 between different directed currents. 1 



1 As a further experimental illustration of the combined axial and 

 orbital revolutions of the solar system, the rotation of the disc of a Gy- 

 roscope affords an instructive example. 



10 



