196 Mr. H. Pender on tlie Magnetic 



that the conditions became so far improved that any attempt 

 at accurate quantitative measurements was deemed worth 

 while. 



At times a deflexion was noticed on reversing the switch S 

 with the disks at rest, but the other parts of the apparatus 

 running. This, however, was invariably traced to the tem- 

 porary failure of the insulation either of the ebonite cores of 

 the disk or of the ebonite between the brushes and the tinfoil 

 conden sing-plates, due to the surfaces becoming greasy. By 

 cleaning the surfaces with benzine and then with alcohol, 

 this deflexion was always completely suppressed. The insula- 

 tion was usually thus cleaned at least once every night, and 

 frequently oftener. No readings were ever taken when there 

 teas any perceptible deflexion with the disks at rest. 



Calcidation of the Effect. 



The strength of the convection-current produced by the 

 rotation of an elementary annular surface of radius r and 

 width dr, carrying a charge of surface- density a is 



dl = 27rrdr— , 



v 



where N is the number of revolutions per second, and v the 

 ratio of the two systems of electrical units. The quantity 

 induced in the coil I (fig. 1) connected with the galvanometer, 

 by reversing this, i. e. by reversing the sign of <7, is 



dq = 4.7rrdr n 



3 v ±1 



where M' is the coefficient of mutual induction of the ring 

 and the coil I, and R the total resistance of the circuit con- 

 sisting of coil and galvanometer. Let the charge a- be reversed 

 C times a second, and the induced current be commutated as 

 above described. The value of the rectified induced current 

 will be 



^nrrdr — • x -rrG, 



and the steady deflexion of the spot of light from its equili- 

 brium position will be 



N/7 



dD=±Trrxdr—xWK, 



v 



where K is a factor depending on the resistance of the circuit, 

 the number of reversals per second, and the sensibility of the 

 galvanometer, or 2M'K is the deflexion given by a unit 

 current reversed in the ring C times a second under the same. 



