394 RECENT PROGRESS IN PHYSICS. 



on A^ more electricity will be induced on B when sliellac is placed be- 

 tween them, than Avhen air is the intervening insulator. The experi- 

 ment indicated in fig. 37 tends just as little as that with the inductive 

 apparatus to lead to the above explanation. 



The following experiment is well adapted to bring the question to 

 a decision : Fio-. 3s. 



Under an insulatedand electrified metallic ball a, 1 or ^ 



2 inches in diameter, fig. 38, place a gold leaf or straw "'"^ 



electrometer at such a distance that a considerable 

 divergence may be obtained. If the ball a be charged 

 with 4-E, then — E will be induced in the plate of 

 the electrometer 6, and the -f E will be repelled by a - X 



into the pendulum ; hence its divergence. 



Now put a plate of shellac between a and h. 



If Faraday's view be correct, a stronger induction 

 must take place through the shellac than before ; 

 more — E should be induced in the electrometer plate, 

 and thus more + E should be forced into the pendu- 

 lum, and its divergence should increase. 



But the experiment shows that the divergence of 

 the pendulum decreases as soon as the shellac plate 

 is introduced. Hence, most decidedly, a stronger 

 induction does not take place through shellac than 

 through air. 



If a greater quantity of the electricity on a is dis uised after the 

 introduction of the shellac than before, it is evidently caused by a 

 mutual action between a and the shellac plate ; but by no means 

 because a stronger induction takes place through the shellac. 



Knochenhauer has instituted an experiment similar to this, but he 

 has entirely mistaken its signification. 



Instead of an electrometer with two suspended leaves, he used a 

 pile electrometer, (Pogg. Ann. LI, 126.) A weak j^ositwe charge 

 was imparted to the conductor a (which, in his experiments, was a 

 metallic plate instead of a ball, producing the same result, however) 

 at the same time the plate of the electrometer was touched ; — E was 

 thus induced in this plate. A plate of shellac was then placed be- 

 tween the electrometer plate and the electrified conductor a, when a 

 movement of the gold leaf took place, and Knochenhauer asserted 

 " that simultaneously with the introduction of the shellac plate the 

 leaf of the electrometer indicated free positive electricity, so that now, 

 on the lower disk^ more negative electricity was disguised." This 

 was in perfect harmony with Faraday's view ; but it is in direct op- 

 position to the results of the experiments with the straw electrometer 

 instituted by me. According to my experiments, I am obliged to 

 suppose that the movement of the gold leaf indicated free negative 

 electricity. 



'It might be supposed that Knochenhauer was deceived as to the 

 pole of his gold leaf electrometer, so that he confounded a negative 

 with a positive indication. 



In order to come at this definitely, I repeated Knochenhauer^ s ex- 

 periment. The ball a was charged with -f- E, and when a shellac 

 plate was introduced between a and the electrometer plate, the gold 



