276 Mr. H. A. Rowland on Magnetic Distribution, 



Here we see an excellent confirmation of the results deduced 

 from Table IV. In Table V., where the magnetizing force is 

 very small, and where, consequently, no part of the iron has yet 



1 R' 



reached its minimum resistance, the value of -5 = -^5- =R'«it de- 



r a 



creases continually as the value of Q' decreases, as it should do. 

 In Table VI., with a higher magnetizing power, which was suffi- 

 cient to bring a portion of the bar to about the minimum resist- 

 ance, we see that -^ remains nearly stationary for a short dis- 

 tance from the helix and then decreases in value. In Table VII., 

 where the bar is highly magnetized and the portion near the 



zero-points approaches the maximum of magnetization, -5 in- 

 creases in value as we pass down the bar ; and having reached 

 its maximum at L = ll J nearly, it decreases.: These Tables, then, 

 show in the most striking manner the effect of the variation of 

 the magnetic permeability of iron upon the distribution of mag- 

 netism. 



It is evident that these Tables also give the data for obtaining 

 the relative values of R' at different parts of the bar ; but the 

 results thus obtained are conflicting, and will need further expe- 

 riment to obtain accurate results. Where such a small* magne- 

 tizing force is used as in Table V. it is almost impossible to 

 attain accuracy; and allowance should be made for this in dedu- 

 cing results from it. The greatest liability to error is of course 

 where the magnetization is small ; for any small residual mag- 

 netism which the bar may contain will be more apparent here — 

 although great care was taken to remove all residual magnetism 

 before use. Resides this there are many other disturbances 

 from which the higher magnetizing powers are free. 



If we accept Green's formula as correct, these observations give 

 us data for determining the magnetizing -function of iron in a 

 unique manner, for nearly all other methods depend on absolute 

 measurements of some kind. Thus the least value of r q in 

 Table IV. for a rod *19 inch diameter is '0142, which gives 

 p = -01132, which in Green's formula (equation 8) gives /z = 3388 

 for the greatest permeability of this iron ; and this is as nearly 

 right as we can judge for this kind of iron. It is to be noted 

 that Green's formula has been found for the portion of the bar 

 covered by the helix ; but, as seen from my formulae, it will ap- 

 proximately apply to all portions, though it would be better to 

 find a new formula for each case. 



"We shall, toward the last, resume this subject again ; and so 

 will leave it for the present. 



