42 Dr. H. Pender on the Magnetic 



it 4 *", M. Cremieu observed a deflexion of the needle when 

 between it and the charged disk there was only a single con- 

 densing plate connected to earth, but when between the 

 needle and this plate a second metallic plate connected to 

 earth was introduced no effect could be obtained. It there- 

 fore seemed worth while to try a similar experiment with 

 the apparatus above described. In this experiment only one 

 disk was used. Between the condensing plate next the coil 

 and the coil itself was introduced a brass plate 1'5 mm. thick 

 connected to earth. On setting the disk in rotation, but 

 without charging it, a great unsteadiness of the needle was 

 noticed. It was discovered that this was due to traces of 

 iron in the brass plate. By gently tapping the plate when 

 the disk was at rest the same effect could be produced. To 

 keep the plate sufficiently steady to make any observations 

 on its shielding effect it was therefore necessary to run the disk 

 at a very low speed. With the disk running at such a low 

 speed, the Voss machine was connected in, and the deflexion 

 of the galvanometer needle observed. Then, without making 

 any other change, the brass plate, connected to earth, was 

 introduced, and the deflexion again noted. This was done 

 several times. The means of a number of readings with the 

 plate out and in were respectively 12*8 and 13'0 mm. de- 

 flexion. From this we can conclude that the introduction of 

 the plate was without any such effect as noted by M. Cremieu. 

 It may be of interest to note in this connexion that an attempt 

 to use a solid aluminium disk in place of the gilded disk in 

 my first experiments was foiled by the magnetic disturbances 

 caused by the traces of iron in the aluminium, although the 

 purest metal obtainable was employed. 



The next experiment tried was the application of Cremieu' s 

 method to the investigation of the magnetic effect of a dielec- 

 tric moving in a uniform electrostatic field, or, in other words, 

 the magnetic action of a moving apparent charge. This 

 question was first investigated by Rontgenf in 1S88. Rontgen 

 showed that a moving polarized disk was capable of deflecting 

 a magnetic needle suspended near it. The maximum de- 

 flexion observed by Rontgen was 3 mms. One objection 

 offered to Rontgen's experiment was that the effect observed 

 might have been caused by the disk assuming a real charge 

 by leakage across from the condensing plates on each side of 

 it. Cremieu's method precludes any such action as this, 

 inasmuch as the condensing plates are rapidly charged and 



* C. JR. cxxxi. p. 797 (1900). 

 f Wied. Ann. xl. p. 93. 



