Euthekfobd. — On the Magnetization of Iron. 503 



Now, this value of H at any point only depends on the 

 current flowing external to that point ; and, since the current 

 is mainly confined to the surface, we may take r = radius of 

 wire. 



2pQ 2 

 H = -^-° = — _pCV , where V is potential between knobs, 



and C is capacity of condenser. 



As the spark-gap was x^in. in length, the difference of 

 potential was as near as possible 10,000 volts. Substituting 

 these values, it will be found that 



XT 1S ' 8 1 



ti — — nearly. 

 For the first wire r = 0-Ollin. = 0-027cm. 



I Q.Q 



.\ H = fr^rz = 1,400 approximately ; 



and, taking B = 12,000, we get a value of 



ju,=-— =:9 approximately. 



The observed value is about 6 ; but the discrepancy between 

 the two results is to be expected, and is due to the fact that 

 the resistance of the iron is measured after a succession of 

 discharges in the same direction, when, on account of the 

 greater amplitude of the first half-oscillation, the inner part 

 of the wire is practically saturated, and does not offer any 

 considerable permeability when once the current has pene- 

 trated through the external skin, magnetized in the opposite 

 direction by the second half -oscillation. The equations 



2J B B?" 



H = — ■ and /a =— =— show that we should expect the per- 

 meability of the iron to vary as the radius of the wire, within, 

 of course, the maximum limit of permeability of iron — i.e., 

 about 3,000. The table given previously shows how closely 

 the law is fulfilled in practice. 



Now, J = pQ = ,— CV , and V varies as the spark- 

 length d, 



T SG 1 



;, J a — = . d, 



V Li 



Br 



and u=— -, and when the iron is saturated B may be taken 



as constant. 



rVh 

 r dVG 

 Therefore increase of radius increases the permeability of 

 iron in these fields, and the shorter the spark-gap the higher 

 the permeability, and therefore the resistance. 



