236 On the Magnetic Effect of Electric Convection. 



the moment of charging the disk AB, would produce on the 

 charging current circulating in ABC.D an electromagnetic 

 effect precisely equal and opposite to the effect of the pondero- 

 motive induction which the rupture of the circuit of the 

 magnet should produce on the charged disk AB. 



The objection would be valid had I used an arrangement 

 corresponding to Mr. Wilson's diagram, reproduced above. 

 In this it is, in fact, assumed that a vertical magnet NS is 

 employed for acting on the charged disk AB. 



But in the various forms of apparatus which I have used, 

 and especially in the last one, of which I have given a detailed 

 drawing*, a closed electromagnetic solenoid was always 

 employed. 



Now it is known that the external field of such a solenoid 

 is zero. Hence no electromagnetic effect on the charging 

 current of the disk AB is possible in my experiment. 

 Mr. Wilson's objection on this point thus falls to the ground. 



The second objection relates to my first repetition of 

 Rowland's experiments, in which I used an arrangement 

 closely resembling his. 



Mr. Wilson thinks that he has traced the cause of my 

 failure to observe the magnetic effect of electric convection 

 to the fixed sectors, which he concludes were insufficiently 

 insulated in my experiments, and well insulated in Rowland's. 



To this I may in the first place reply that since the above 

 experiments 1 have carried out two others f, in which the 

 fixed sectors were completely suppressed, and the results of 

 which have confirmed, those of the preceding experiments. 



Mr. Wilson's second objection has therefore no foundation. 



But this objection is further based on an altogether gra- 

 tuitous assumption. 



In studying the papers of Rowland, Rowland and Hutchinson, 

 and finally Himstedt, I was led to suspect that they had not 

 paid sufficient attention to the insulation of their sectors, 

 either fixed or movable. I therefore used especial care in this 

 connexion. The insulation of each sector was separately 

 tested. In the case of the movable disk, it was tested when 

 the disk was both at rest and in motion. I thus made sure 

 that no charge was transferred from the previously charged 

 sector to the neighbouring sectors on account of rotation. 



Besides, the distance between the sectors was from 8 to 

 20 millimetres, according to the experiment ; and the ebonite 

 or mica which separated them had been carefully cleaned and 

 varnished with shellac dissolved in absolute alcohol. 



* Comptes Rendus, vol. cxxxi. p. 578 (1900). 

 f Ibid. vol. cxxxii. p. 1108 (1901). 



