48 ELEMENTS OF STATIC ELECTRICITY. 



positive and B negative, is attracted by B ; but the 

 central ball, being neutral, remains unmoved. 



If the sphere, A, be negatively electrified, these condi- 

 tions will all be reversed. Electricity will be attracted 

 to the end of C next A, and a positive charge, equal 

 to the negative on J., attracted from the earth to B. 

 Hence the balls will assume the same positions as 

 before. 





Fig. G Cylinder Electrified by Induction. 



Similar inductive effects can be produced on the cyl- 

 inder by the sphere A alone, but less marked than when 

 two spheres are used; and, for such an experiment, tin- 

 foil electroscopes are better than those made with pith 

 balls, being more sensitive. 



THEORY OF INDUCTION. It Is noc known how in- 

 ductive force is transmitted. The hypothesis has been 

 advanced that it is by a certain strain of the medium ; 

 as when a weight is lifted by a rope or pushed by a pole, 

 the energy is transmitted in one case by the tension of 

 successive portions of the rope, and in the other by a 

 compression of successive portions of the pole. In 

 either case the energy or stress produces a strain, which 



