Magnetic Permeability of various Alloys of Iron. 105 



The magnetometer readings multiplied by a factor give the magnetic induc- 

 tion B. This factor vai'ies with each specimen, as it depends on the diameter of 

 the rod under test and the directive force F at the magnetometer ; the length of 

 the rod, 102 cms., is the same in all cases, and the distance between the jDoles 

 was, in general, 95 cms. The magnetometer scale was such that each division 

 corresponded to an angle whose tangent was = 0'0227, i.e. tan 6 = 0'0227 

 X magnetometer reading. As B = 47rl + H, and tlie intensity of magnetisation 



_ ir^F tan 



Trd' 1 - ( ^ 



■'rVi' 



[-. 



wliere r = distance from upper pole of rod to magnetometer, 

 r' = ,, ,, lower ,, ,, ,, 



d =: diameter of rod in cms., 

 F = directive force at magnetometer, 

 6 = angle of deflection of magnetometer needle, 



we thus get the value of I jDcr scale division of the magnetometer. Inserting the 

 actual values for the particular specimen given (1179 B) in Table I., we have 



^ 4 X 45^ X -175 X -0227 ^„ , 



1 = ; = OO'i. 



73- X -54- X -921 



Taking the highest current given in column 2, Table I., viz. 1"5, we have 



H = 1-5 X 30-2 == 45-3 ; 



as the magnetometer reading for this magnetising force was 30'0, we have 



I = 38-1 X 30-0 = 1143, 



therefore B = 477 x 1143 + H = 14400. 



This gives the maximum induction of this particular chromium-aluminium steel 

 for a magnetising force of 45*3 C. Gr. S. units. The rctcntivity .^ after this force, is 

 shown b}' 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, — i.e. when the current is 

 between O'lS and 0-31 ampere, exactly 0-265 ampere. This current x 30*2 (the 

 magnetic force of the coil per ampfere) = 8 C.Gr.S. units. 



