246 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



in which the /mean is the magnetization as determined magnetometri- 

 cally, and the /max. is found from the B as determined ballistically 

 at the centre of the rod in the usual way. For this rod m = 300. 

 The abscissae represent H', the magnetic field applied from without. 

 Similar curves had also been previously published by Dr. L. Holborn,^! 

 only the susceptibilities were taken directly from the unsheared mag- 

 netization curve of a short cylinder. 



Although these experiments of Holborn and Benedicks practically 

 prove the increased uniformity of magnetization for low fields, it is 

 perhaps a better plan to settle this point by a more direct method. It 

 was therefore thought that it might be of interest to compare the 



10 20 30 40 50 60 70 60 90 iOO 110 120 130 140 150 160 170 1S0 130 200 



Figure 21. 



Benedicks's curves, showing variation of the pole-distance ratio and the suscep- 

 tibility in an iron rod. The abscissae give the field H' in c. g. s. units. 



actual magnetic induction which passes through various cross-sections 

 of some of our iron rods, for practically the whole range of magnetiza- 

 tion from zero to saturation. To do this one might use a secondary 

 search-coil, fitting loosely around the iron rod, which can be suddenly 

 displaced along the rod by any desired distance. This would require 

 two observers ; but it could not be used conveniently in this work 

 since the rods in which the magnetic induction was tested were 1.905 

 cms. in diameter, and the inner diameter of the brass tube around 

 which the solenoid coils were wound was not much larger. Another 

 method would be to wind coils around different parts of the rod and 

 get the actual induction passing through each coil. This would do 



21 Sitzber. Akad. d. Wiss., Berlin, 1, 159-162 (1808). 



