454 



Messrs. V. Cremieu and H. Pender 



on 



of any point on the ring NRMP. Suppose SSj charged to a 

 negative potential K, and NKMP connected permanently to 

 the earth. As each element ds of NRMP arrives opposite S 

 it will become charged by influence. If the hypothesis above 

 stated is true, this element should carry its charge along 



Fio-. 3. 



with it. In other words, this charge will be displaced with 

 reference to the electrostatic field between NRMP and SSi, 

 which remains fixed in space. When ds arrives opposite Si, 

 the charge on it is no lonoer retained bv influence. As this 

 is true for all elements of NRMP, there will result from the 

 movement a constant diflference of potential between the two 

 points N and Ni on the moving disk opposite S and Si, con- 

 sequently conduction-currents will be set up in the ring, and 

 will be distributed according to the relative resistances of the 

 sectors NRNi and NPNi. 



A magnetic needle placed above R will therefore be sub- 

 jected to a magnetic field due to two causes : first, that due 

 to a convection-current in the direction of the arrow ; and 

 second, that due to a conduction- current in the opposite 

 direction. If the needle is placed over P, it will be acted 

 upon by the conduction-current only. Moreover, the sum 

 of the conduction-currents in NRNi and NPNi should be 

 equal to the convection-current. 



Let (T be the surface-density of the charge on NRMP, 

 then the intensity of the convection-current due to a ring 

 1 cm. wide is 



I = cry. 



But 



(7 = 





