H. A. Rowland—Studies on Magnetic Distribution. 3381 
diately surrounding the rod. The resistance of such an en- 
velope per unit of length of rod is 
1 en D 
Qn fl, TP ee 
where D is the diameter of the envelope, d of the rod, and pz, 
the permeability of the medium. We are not, however, able to 
estimate D. If, however, we have two magnetic systems similar 
in all their parts, it is evident that beyond a certain point simi- 
larly situated in each system we may neglect the resistance of 
the medium, and — will be the same for the two systems. 
Hence R’ is approximately constant for rods of all diameters in 
the same medium, and r takes the form 
2 1 
| rome Se Oe ae eee ee ee © 2 
av sae (7) 
It is evident that the reasoning would apply to rods of any sec- 
tion as well as circular. 
Green’s splendid essay (reprint, p. 111, or Maxwell’s 
Treatise on Electricity and Magnetism,’ art. 489) we find a 
formula similar to equation (5), but obtained in an entirely dif- 
ferent manner, and applying only to rods not extending beyond 
the helix. e ‘Reprint’ 6 corresponds to my r, and its 
value, using my notation, is obtained from the equation 
'231863—2 hyp. log p+2p= (8) 
rd 
< 
If we make u a constant in this formula, we must have 
4 
(u—1)p?” 
where p = 
me j 
p= > = constant, hence roc—, 
IIL. 
_Among the various methods of measuring linear magnetic 
distribution, we find few up to the present time that are satis- 
tory. Coulomb used the method of counting the number of 
vibrations made by a magnetic needle when near various points 
of the magnet. Thus in the curve of distribution most often 
reproduced from his work, he used a magnetized steel bar 27 
