

The Followers of Maxwell. 339 



translatory motion of electric charges. That the convection 

 of electricity is equivalent to a current had been suggested 

 long before by Faraday.* "If," he wrote in 1838, "a baU 

 be electrified positively in the middle of a room and be then 

 moved in any direction, effects will be produced as if a current 

 in the same direction had existed." To decide the matter 

 a new experiment inspired by Helmholtz was performed by 

 H. A. Kowlandf in 1876. The electrified body in Kowland's 

 disposition was a disk of ebonite, coated with gold leaf and 

 capable of turning rapidly round a vertical axis between two 

 fixed plates of glass, each gilt on one side. The gilt faces 

 of the plates could be earthed, while the ebonite disk received 

 electricity from a point placed near its edge ; each coating of 

 the disk thus formed a condenser with the plate nearest to it. 

 An astatic needle was placed above the upper condenser-plate, 

 nearly over the edge of the disk; and when the disk was rotated 

 a magnetic field was found to be produced. This experiment, 

 which has since been repeated under improved conditions by 

 Kowland and Hutchinson,J H. Fender , and Eichenwald,|| shows 

 that the " convection-current " produced by the rotation of a 

 charged disk, when the other ends of the lines of force are on an 

 earthed stationary plate parallel to it, produces the same mag- 

 netic field as an ordinary conduction-current flowing in a circuit 

 which coincides with the path of the convection-current. When 

 two disks forming a condenser are rotated together, the 

 magnetic action is the sum of the magnetic actions of each of 

 the disks separately. It appears, therefore, that electric charges 

 cling to the matter of a conductor and move with it, so far as 

 Rowland's phenomenon is concerned. 



The first examination of the matter from the point of view 

 of Maxwell's theory was undertaken by J. J. Thomson,1[ in 1881. 

 If an electrostatically charged body is in motion, the change in 



* Exper.Re*., 1644. 



t Monatsberichte d. Akad. d. Berlin, 1876, p. 211 : Ann. d. Phys. clviii (1876), 

 p. 487 : Annales de Chim. et de Phys. xii (1877) p. 119. 



i Phil. Mag. xxvii (1889), p. 445. \ Ibid, ii (1901), p. 179 : v (1903), p. 34. 

 || Ann. d. Phys. xi (1901), p. 1. H Phil. Mag. xi (1881), p. 229. 



Z 2 



