} 
, 
Magnetization of Iron in Bulk. bau 
In curve I, there is scarcely any initial pause, which is to 
be expected, since the core starts well on the way to saturation. 
It is of some practical importance to notice that the 
horizontally shaded areas abo, aco represent the magnetism 
which the current induces whilst rising. 
Hquation (1) may be written 
dN =(e—12)dt ; 
so that 
PY Ge aa. We, aera) 
Tay 
where i, is the final steady current. 
The value (¢)—7) is the intercept between the curve and 
the horizontal tangent. ‘The right-hand side therefore is the 
area A between the curve, the vertical axis, and this tangent. 
Thus 
Nee HOA Saye 4) d,s (6) 
k’ being the volt seconds per sq. cm. of the record, known 
from the scale of the instrument. This is independent of 
whether the core is solid or laminated. 
If A,, A, are the respective areas abo, aco, c the cross- 
sectional area of the core, and ¢ the turns on the winding, 
then the change of flux density corresponding to an area is 
N kA 
tai earn Fe eee CG) 
Denoting by B,, Bz, the flux densities corresponding to 
A,, A», the following relations are evident :— 
B,+B : ae el 
\_—* =B,, the maximum induction, 
B,—B : , 
arty = B,., the residual magnetism. 
Thus B, +B, Bn (8) 
Bo By ee Lor 
ake ee aa (9) 
k 
ue 
A,— Ai =B,~, nurs : : . . (19) 
The sum of the horizontally shaded areas therefore repre- 
sents the maximum induction density reached, their difference, 
that is the vertically shaded area forming the loop bco, the 
residual magnetism. It is clear that if there were no reten- 
tivity the curves would be identical. 
