Effect of Electrical Convection. 181 



Lecher* repeated Rowland's experiment, but obtained nega- 

 tive results. Only a short account of this investigation was 

 published, no details of the apparatus being given. It is 

 impossible, therefore, to say why he obtained no effect. It 

 may have been because his apparatus was not sufficiently 

 sensitive. 



In 1885, in an attempt to detect the existence of displace- 

 ment currents, Rontgen f had occasion to use an apparatus 

 similar to that employed by Professor Rowland in his Berlin 

 experiment. The ebonite disk in this case was not covered 

 with gold. Incidentally, the effect on the suspended needle 

 of charging the disk, when rapidly rotating, by a brush 

 discharge from a series of points, was tried. The Rowland 

 effect was readily observed. On reversing the sign of electri- 

 fication, a deflexion of from 8 to 10 mm. was noted. In 

 1888 % t m ' s apparatus was again used to investigate the action 

 of a dielectric moving in a homogeneous electrostatic field. 

 If the upper condenser-plate is charged negatively (say) 

 and the lower positively, then there will be an apparent 

 positive charge on the upper surface of the disk, and an 

 apparent negative charge on the lower surface. If then the 

 motion of an apparent charge due to the polarization of the 

 dielectric produces magnetic action, the needle, which was 

 nearer the upper than the lower surface, ought to show a 

 deflexion on rotating the disk and charging the condenser 

 formed by the two stationary plates. After all precautions 

 had been taken to prevent the disk from assuming a real 

 charge, a deflexion of from 2 to 3 mm. was observed on 

 reversing the sign of electrification of the condenser-plates §. 

 On increasing the difference of potential between the con- 

 denser-plates until a brush discharge took place between the 

 upper plate and the disk (the upper plate was *14, the lower 

 "2b cm. from the disk), the Rowland effect was again readily 

 observed. 



In 1889 Professor Rowland and C. T. Hutchinson || re- 

 peated in this laboratory the original Berlin experiment, 

 employing, however, a different form of apparatus. Instead 



* Rep. d. Rhys. xx. p. 151 (1884). 



t Sitzb. (I. Bert. A/cad. 1885, p. 95. 



% Ibid, 1888, p. 23 ; Wied. Ann. xl. p. 93. 



§ Rontgen gives no calculation of the effect which should be exppcted. 

 On the assumption that the magnetic force due to any element of surface 

 is proportional to the quantity of apparent charge passing a given point 

 in unit time, I have calculated from the data given in Rontgen's paper 

 that the detiexion should be 2*1 mm. T employed the formula} used by 

 Rowland in his Berlin experiment (loc. cit). 



|| Phil. Mag. [5] xxvii. p. 445. 



