Magnetic Permeability of various Alloys of Iron. 105 
The magnetometer readings multiplied by a factor give the magnetic induc- 
tion B. This factor varies with each specimen, as it depends on the diameter of 
the rod under test and the directive force / at the magnetometer; the length of 
the rod, 102 ems., is the same in all cases, and the distance between the poles 
was, in general, 95 cms. ‘The magnetometer scale was such that each division 
corresponded to an angle whose tangent was = 0:0227, 7c. tan 0 = 0°0227 
x magnetometer reading. As B=47I +H, and the intensity of magnetisation 
47°F tan 6 
/ y 3 
ally _f ZW 
ard if i 
where 7 = distance from upper pole of rod to magnetometer, 
d/ — ” 
ie ” ,, lower ,, ” ” 
d = diameter of rod in cms., 
F = directive force at magnetometer, 
7) 
angle of deflection of magnetometer needle, 
we thus get the value of I per scale division of the magnetometer. Inserting the 
actual values for the particular specimen given (1179 B) in Table I., we have 
_ 4x 45? x 175 x 0227 | 
I = 30°. 
a xX °54* x -921 3 
Taking the highest current given in column 2, Table I., viz. 1:5, we have 
130 == 105) S< UR Se 4559S} 8 
as the magnetometer reading for this magnetising force was 30:0, we have 
I = 38:1 x 30-0 = 1143, 
therefore B 47 x 1143 + H = 14400. 
This gives the maximum induction of this particular chromium-aluminium steel 
for a magnetising force of 45°3 C.G.S. units. The refentivity, after this force, is 
shown by the residual induction in the cycle when the current is zero. From 
columns 4 and 5 in Table I., we see that the mean magnetometer readings on 
either side of zero is 19°85 for zero current. Working out this value as above, we 
get B = 9500. The coercive force of this steel, in terms of H, is found where the 
curve cuts the axis of H: here the induction is zero. From Table I., it will be 
seen that this point lies between the steps up 2 and 3,—?.e. when the current is 
between 0°18 and 0°31 ampére, exactly 0:265 ampére. This current x 80:2 (the 
magnetic force of the coil per amptre) = 8 C.G.S. units. 
