244 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Now taking ]Sf= 0.0217 for m — 40, we have 



H=H' - NI=ff' - KB 



and K = N/A tt, since we may neglect II in comparison with B. We 

 get, therefore, 



^=0.00173, 



and may now calculate /f and the other quantities from the B of the 



above table. This gives us 



We chose the value of N as would correspond to the ballistic step 

 method. Had we, however, used the method of reversals with a 

 solenoid wound on a pasteboard tube, or a split brass tube, then the 

 ballistic throws observed would have been a little more than twice as 

 great as those we found. If we take them as exactly twice as great, 

 and if we assume that the time-constant of the solenoid is the same as 

 fop the short solenoid used in the earlier half of this work, then we 

 should have 



N = 0.0206 



A" =0.00164 



and the calculated values of the demagnetizing fields, the resultant 

 fields, and the permeabilities would be : 



