RECENT PROGRESS IN PHYSICS. 395 



leaf moved towards the positive jjole of the instrument; that is, to- 

 wards that one which it strikes when a rubbed stick of rosin is ap- 

 proached from above ; the gold leaf then received free — E by inserting 

 the shellac plate. 



If Knochenhauer s view were correct, the gold leaf, on the introduc- 

 tion of the shellac plate, {a being positive.) should move toward the 

 side whicli is approached when a glass rod rubbed with silk is brought 

 over the electrometer. But the proximity of rubbed glass produces a 

 result opposite to that effected by the presence of the shellac plate. 

 Thus the error of Knochenhauer with reference to the nature of the 

 indication is proved. 



Faraday's opinion that a stronger induction is effected through 

 shellac than through air, is hence decidedly wrong. The experiments 

 made with the straw electrometer, as well as with the pile electrome- 

 ter, directly contradict this view. 



But how are all these phenomena to be explained ? I beg leave to 

 offer a few hints, which, perhaps, will serve to point out the way that 

 may lead to a definitive decision of the question. 



If we introduce between the electrified ball a, fig. 38, and the straw 

 electrometer an uninsulated conductor, the pendulum will collapse. 

 According to known laws, nothing else could be expected. 



If we introduce an insulated metallic disk between a and the elec- 

 trometer, a considerable diminution of the divergence will occur, but 

 the pendulum will not completely collapse. This is in consequence 

 of an inductive action which a exerts upon the intervening insulated 

 metallic i)late. 



If we introduce a shellac plate between a and the electrometer, a 

 similar diminution of the divergence will take place, but yet not so 

 much as in presence of the insulated metallic plate. This seems to 

 indicate that the electrified body a causes an induction even in the 

 shellac, though not to such an extent as in a good conductor. In 

 fact, we know that shellac, though a very bad conductor, is not an 

 absolute insulator. 



Knochenhauer also seems to hint at something similar in the me- 

 moir cited. At any rate, this matter needs further investigation ; 

 but so much is certain, that a more powerful induction does not take 

 place through solid insulation than through air, as Faraday main- 

 tains. 



§ 23. Induction in curved lines. We proceed now to the consider- 

 ation of Faraday's proof of induction in curved lines. 



A cylinder of shellac 0.9 of an inch in diameter, which 

 can be j)laced upright, and has a cavity at the top, is 

 electrified by friction, and a brass ball 1 inch in diam- 

 eter laid in the cavity or cap. If now an insulated proof 

 ball be brought into the positions indicated by d, c, h, 

 and e, touched for an instant, and then tried whether it 

 have any electrical charge,, and of what kind, it is found 

 that the carrier will receive a positive charge at d and c, 

 as well as at b and e. 



The result of this experiment has nothing in it at all 

 remarkable, it might have been predicted. The ball B 

 is electrified positively by induction ; the — E of the shellac cyiiuder 



