DYNAMICAL PRINCIPLES TO PHYSICAL PHENOMENA. 
325 
or considering only the term — I 3 /2B in the kinetic energy we have 
i I2 | e g+w(e+/+^) + ^(e-/~^) = °.(44) 
Now Joule found that the volume of the bar was not altered, so that e-{-f-^-g = 0 
and the last equation becomes 
* Is i(g)+ 2Be =° • ■ • ..< 45 > 
or “=i|(-g). (46) 
as the bar lengthens when it is magnetised e is positive, and thus — 1/B increases 
with e. 
Going back to equation (29) we have, if H be the external magnetic force, 
C('ifr".< 47 > 
or as it may be written 
I I=(I)= H . (48) 
Let us suppose that the magnetising force H remains constant, then I will increase 
if — d(F/B)/c/P diminishes, and diminish if this quantity increases. If the change in 
this quantity is due to an increase Se in the elongation, equation (48) shows that the 
corresponding increase 81 in the intensity of magnetisation will be given by the 
equation 
.( 49 ) 
dI 3 \By 
hut if k be the coefficient of magnetic induction 
vk-IGD. (50) 
by equation (48); so that 
si =-!S Se . (51) 
MDCCCLXXXV. 2 U 
