﻿of Electricity on Iron Wires. 4»i) 



By the use of Lord Rayleigh's formula for inductance 



under very rapid oscillations, it is easy now to calculate a 

 value for the permeability of the iron. 

 Lord Rayleigh's formula is : — 



L '=< A V|). 



where / is the total length of circuit, A a constant depending 

 only on the form of the circuit, or ZA is the inductance of a 

 similar copper circuit ; jjl the permeability ; R the ohmic 

 resistance ; p = 27rn, where n is the number of complete 

 oscillations per second. 



The value of j) = '27rn = 36 x 10 7 . 



E for iron wire (diam. O'll 86 centim.) = '1328 ohm per metre. 

 „ „ „ ( „ 0-08847 „ ) = -227 „ 

 „ „ „ ( „ 0-0785 „ )=-301 „ 



For iron (diameter 0-1136 centim) : — 



I/-1-0ML-I(A+Vg). 



L + -034L=L + ^g, 



■034 L = ^g. 



Calculating the value of L for a similar copper circuit I 

 units long, substituting the value in the above equation, and 

 solving for the three cases, we get : — 



For the iron wire, diameter 0*1186 centim., /^ = 430, 



0-08847 „ ,1=389, 

 „ „ „ 0-0785 „ /* = 336. 



These values for the permeability all fall within a reasonable 

 limit, and have for an average /x = 385. Those are the values 

 found for different specimens of wire made by the same 

 company, but the specimens were wound and unwound and 

 stretched many times during the series of observations. 

 Besides the shortening of the w r ave-len£th there is shown a 

 a decided increase in the damping, as has already been 

 observed by Trowbridge and Bjerknes. In fig. 4 (PI. XII.) 

 the curves for iron fall below the corresponding ones for 

 copper, but owing to the change in the activity of the spark 

 no exact measurement was made. It was only observed that 

 the bolometer throws with the copper circuit were always 



