Magnetic Permeability of various Alloys of Iron. 125 
Hopkinson, and probably than that used by Ewing. In a specimen of very pure 
iron from the Elswick works, the maximum induction in the same field as ours, 
45 C.G.S., is given as 17450 by Messrs. Lydall and Pocklington in the Proceedings 
of the Royal Society, vol. 52, p. 228. From tests made on a sample of Hadfield’s 
dynamo ‘‘ steel,” Prof. Ewing gives the maximum induction at 45 as 16900, more 
complete tests at Finsbury College, with a ring of the same steel, gave the follow- 
ing values :— 
Maximum induction, . . 16480, for H = 45-5. 
Permeability, | : >; L@eod, La = OO! 
Coercive force, . ; OM OOs 
Loss in ergs per cycle, . . 12080, on 9g == OL 
The correction for the demagnetizing action of the ends raises the residual 
induction and permeability in our specimen of iron, as will be seen in Note A 
here appended. 
In conclusion, we desire again to express our thanks to Mr. Allen and 
Mr. Wills for frequent assistance in the numerous observations required in the 
second part of this paper. 
Nore A. 
As we have already pointed out, a correction is necessary for the demagnetizing 
reaction of the ends in our rods, especially in those of high permeability. Prof. 
J. A. Ewing, F.r.s., has shown (Pil. Trans., 1885, Part 1, p. 536) that ‘ even 
when dealing with the softest iron, we may take a rod, whose length is not less 
than 300 diameters, as giving results scarcely different from those given by a ring 
or longer rod”: and, ‘‘in hard iron and in steel a smaller ratio of length to dia- 
meter would, no doubt, give an equally good approximation to the condition of 
endlessness.” In his work on Magnetic Induction in Iron and other Metals, p. 31, 
Ewing gives a table of correcting factors for rods of various lengths. The rods 
we have tested are 200 diameters long, and, according to Ewing, the actual mag- 
netizing force is the original force in the solenoid, diminished by 0:000125 times 
the induction for any given point. Thus, if we take a point on the B-H curve 
of our annealed iron (this curve is repeated in each plate), where B= 16000, we 
find H=16°5; then the actual value of H for this induction will be 16:5 — 2 =14°5, 
since 0:000125 x 16000 = 2; and so on for any other point on the curve. It there- 
fore follows that the values of H must not be measured from the vertical axis OB, 
but from a line inclined to the right, and drawn through O to a point whose 
co-ordinates are B= 16000 and H=2: this line will make an angle of two degrees 
with OB. It is easy to draw such a line on our plates, by placing a straight-edge 
