182 Trowbridge and Adams — Circular Magnetization 



giving, 



n = 1423 

 This value of n, with the calculated value of the self-induction 

 of the coil, gives as the capacity of the condenser at this fre- 

 quency : 



C = 1*756 micro-farads. 



Discharging the condenser through the smaller coil and the 

 iron wire in series, we had : 



Length of half-oscillation 1 1 *85 cm 



Speed of mirror 8*0 



Distance plate from mirror 300*9 



giving 



n = 1280 



Using 1*756 as the capacity of the condenser, and this value of 

 n, we get for the combined self-induction of coil and iron, 



L = 8*805 X 10 6 

 Subtracting the self-induction of the coil, that of the iron wire 

 becomes : 



L' = 1-68X10 6 



And by means of Rayleigh's formula, 



//, = 443 * 



Discharging the condenser through the iron wire alone, we 

 had: 



Length of half-oscillation 2*42 cm 



Speed of mirror . 4 2 



Distance of plate from mirror 300*9 



giving 



n = 3300 



Assuming the capacity of the condenser to be 1*668 micro- 

 farads, and using this value of the frequency, we get for the 

 self-induction of the iron wire : 



L'= 1*395 X10 6 

 By means of Rayleigh's formula, 



fJL= 711 



The values of the permeability so calculated are a kind of 

 mean value for a single discharge of the condenser. The mag- 

 netic force is that due to the current in the wire, so that the 

 iron becomes circularly magnetized. This magnetic force is 

 at the axis of the wire, and increases to the value 2i/a on the 

 circumference, where i is the current and a the radius of the 

 wire. Thus the magnetic force is not a constant over the 

 cross-section of the wire ; at any point in the wire its value is 

 27rir/a' i , where r is the distance of the point from the axis of 



