•2il PROCEEDINGS OF THE AMERICAS ACADEMY. 



Now taking N — 0.0217 for m = 40, we have 



// = //' _ m = //' - kb 



and A'= N/4 ir, since we may neglect // in comparison with />'. We 

 therefore, 



K =0.00173, 



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

 above table. This gives as : 



We chose the value of X 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 

 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 

 for tlic short solenoid used in the earlier halt' of this work, then we 

 should have 



X — 0.0206 



A' = 0.00164 



and the calculated values of the demagnetizing fields, the resultant 

 fields, and the permeabilities would he : 



