108 HENEY A. ROWLAND 



points approaches the maximum of magnetization, a increases in value 



as we pass down the bar; and having reached its maximum at L= 11 

 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 magnetism. 



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 experiment to obtain 

 accurate results. Where such a small magnetizing force is used as in 

 Table V it is almost impossible to attain accuracy ; and allowance should 

 be made for this in deducing results from it. The greatest liability to 

 error is of course where the magnetization is small; for any small re- 

 sidual magnetism which the bar may contain will be more apparent 

 here although great care was taken to remove all residual magnetism 

 before use. Besides 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-f unction of iron in a unique manner, for 

 nearly all other methods depend on absolute measurements of some 

 kind. Thus the least value of r z in Table IV for a rod -19 inch diam- 

 eter is -0142, which gives p= -01132, which in Green's formula (equa- 

 tion 8) gives //=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 formulse, it will approxi- 

 mately 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. 



The results which I have now given, and indeed all the results of this 

 paper, have been deduced not only from the observations which I pub- 

 lish, but from very many others; so that my Tables may be considered 

 to represent the average of a very extended series of researches, though 

 they are not really so. 



V. 



Let us now consider the case of that portion of the bar which is 

 covered by the helix. First of all, when the helix is symmetrically 

 placed on the rod, equations (5) and (6) will apply. As Q" is the 



