and Magnetic Permeability. 179 



of the Jefferson Physical Laboratory — 200 feet long and 60 

 feet wide. The wire was the ordinary soft iron wire, 0*105 

 inches in diameter. This iron wire was arranged so that it 

 could be put in series with either of the two copper coils, and 

 also so that it could form practically the only self-induction in 

 the discharge circuit. 



The general method of procedure was as follows : As stated 

 above, one of the copper coils was placed at A and the 

 capacity of the condenser obtained. Next the iron wire was 

 placed in series with the same copper coil. This of course 

 changed the frequency of the discharge, but since the change 

 was not large, it was assumed that the capacity of the con- 

 denser was not sensibly altered. Knowing the periodic time 

 and the capacity, the self-induction of both copper coil and 



iron wire was given by the formula T = 27r\/LC. Then the 

 self-induction of the iron wire alone was obtained by subtract- 

 ing from this combined self-induction, the calculated self- 

 induction of the copper coil. Then by means of Lord 

 Rayleigh's formula : 



' L '='(Wi~) 



the permeability of the iron was obtained. 



The same process was repeated using the other copper coil, 

 and the permeability at this different frequency obtained in 

 the same way. Finally the iron was placed alone in the dis- 

 charge circuit, and the self-induction of this obtained directly 

 from the formula : T = 27r\/LC. In order to get the capacity 

 of the condenser at this highest frequency, it was assumed 

 that it had diminished by 5 per cent. This, of course, can 

 only be regarded as a rough approximation. 



The experimental data and calculations follow : 



The self-induction of the two coils of copper wire was calcu- 

 lated by means of Maxwell's formula : 



where : 



4Trn 2 a(\og^- — 2)+2l(\og-y + 0'11835j 



n number of turns. 



a mean radius of coil. 



R geometric mean distance of cross-section of coil. 



I total length of wire. 



D diameter of insulated wire. 



d diameter of bare wire. 



