( 660 ) 
Now, the energy of a volume-element dv of the ellipsoid which 
we consider to be very long and magnetized with equal intensity 1 
in the direction of the field, is 
W = — IHdv 
and therefore the moment of the couple exerted by the field on this 
element is 
dW 8 
dM'. 
00 2 
The couple exerted by the field H 0 on the ellipsoid is 
M — {N y — NJ Pv sin ip cos ip. 
In very strong fields the condition is fulfilled that the magnetization 
is parallel to the external field nothwithstanding the demagnetizing 
forces of the ellipsoid, and therefore 0 = ip. 
Hence the disturbing moment dM ' varies with azimuth of the 
substance in exactly the same manner as the chief couple M. The 
maximum value of the couple dM' is 
dM 1 ~ dv I. 
which for the whole ellipsoid gives 
'-S'G 
where a is the semi-major-axis of the ellipsoid. 
Let us now make the assumption that the field changes confor¬ 
mally, and let us call the field 1cm. from the axis in the direction 
of the y-axis (1 — *?) H 0 , then 
rr 1 /d 2 tf\ 
and the ratio between the maximum values of the couples is 
— = 
M 5 * 
therefore ’ the ^ond 
“ r f,H :" S C0UP ' e inCreaSe3 Proportionally to the 
strength of the field. I„ the IH diagram, a sloping instead of a 
