ELECTRICAL WAVES ARE SOME OE ITS APPLICATIONS. 
of a small mirror mognefcometer. The needle was either fixed in position in the 
solenoid and the magnetometer placed beside it, or the needle removed and tested 
after each experiment. If wires of the same length but of different diameters be 
taken, it will be found that the magnetization is roughly proportional to the diameter 
of the wires. This is to be expected, since the magnetizing forces are confined to a 
thin skin on the surface of the needle, and so the amount of magnetization depends 
more on the surface than on the sectional area. 
In order to determine accurately the way in which a piece of steel was magnetized 
from the surface inwards, recourse was had to a method of solution of the iron in acid. 
The needle to be tested was fixed in a glass vessel before a dead-beat magnetometer. 
Dilute hot nitric acid was poured in and kept at a constant temperature. As soon 
as the needle was covered it commenced to dissolve, and the variation of the deflection 
with the time was noted. In this way the amount and stages of the magnetization of 
the iron could be completely determined. From preliminary exjDeriments on 
uniformly magnetized needles, it was found that under the action of the acid the 
diameter of the wire decreased uniformly v/itli the time. 
Let I represent the intensity of magnetization of a thin circular shell distant r 
from the centre of the needle, and M the deflection of the magnetometer at any 
instant. 
I. '2TTrdr is proportional to M, 
► 0 
therefore 
In is proportional to rZM / dr. 
Let r be the radius of the wire at first. It has been shown that a —• r is 
proportional to t, the time of action of the acid. 
Therefore 
o — r = Kt where k is a constant, 
and 
— dr — ndt, 
therefore 
. . 1 dU 
i vai ies - - . . 
a — ict at 
If a curve be plotted whose ordinates represent the deflection and the abscissm 
the time of action of the acid, dWjdt at an}^ point is equal to the tangent of the angle 
which the tangent to the curve at that point makes with the axis of x. The variation 
of I can thus be completely determined from tlie experimental curve. 
lire following curve (Curve I) is an example of the magnetization of a piece of 
pianoforte wire, 4 centims. long, ‘08 centim. in diameter, placed in a solenoid of two 
turns per centim. The frequency of the discharge was about 3 million per second. 
B 2 
