Permeability of Iron and Steel. 81 
justified in leading to values of w independent of the dimen- 
sions of the rings. 
It is a question of particular interest whether the iron in 
the interior of thick bars is in any degree shielded by that 
which encloses it from the effects of magnetic potential. In 
rings we have this question detached from the complications 
caused by the ends of straight bars or tubes, and are face to 
face with the simple question:—Is magnetic potential so ab- 
sorbed in its passage through external layers of iron that its 
effect in the interior is sensibly diminished ? 
To answer this question it is convenient to make use of the 
e e e e si ° 
ordinary expression for magnetizing force (= — in the above 
tables), and to tabulate the values of the induction in the dif- 
ferent rings corresponding to a series of definite values of the 
magnetizing-force. If, then, there is absorption of the force 
in the external layers, the inside of the thickest rings should 
be the most shielded, and the induction should throughout be 
lower in those rings for the same magnetizing force. 
An inspection of the following Table will show that, if any- 
thing, the reverse of this is the case. 
Table of Values of % in the different Crown Iron Rings, cor- 
responding to certain Values of the Magnetizing Force. 
G EH a) H. K 
Mean diam....| 21:5 em. | 10°035 ecm. | 22:1 em. | 10°735 em. | 22°725 em. 
Bar-thickness.| 2°535 1-298 1:292 alten igs "7544 
Maenetizing 
Force. 
, 126 73 65:3 82:4. 85 
D Beit 270 224 208 214 
u 1449 1293 840 675 885 
2 4564 3952 Dao 2777 2417 
5 9900 9147 8293 8479 8884 
10 13023 13357 12540 11376 11388 
20 14911 14653 14710 14066 13273 
50 16217 15704 16062 15174 13890 
100 17148 16677 17900 16134 14837 
The thickest ring, G, has the highest inductions throughout. 
The thinnest rings fall a little behind on the whole, except in 
the initial values. It seems probable that this may be due to 
slight flaws of small surface-extent, which may not percep- 
tibly impair the magnetic conductivity of the thick ring, but 
may interfere with that of the thin ring. At all events the 
hypothesis of shielding is conclusively negatived. 
Returning to the general tables, the variations which strike 
