286 
MR. G. T. WALKER ON REPULSION AND ROTATION 
7 27 r~p i a i a' i (Ta' 
K ~pY 2 \A\d? L‘ 
am 
2 (y — y) 4* 3 sin 2 (a — y - y')\ 
where K cos pt is the magnetic moment of unit length. 
To this order then 
(a.) If the couples on the shells be equal and opposite 
a 
y y — o, or 
(f3 .) The couples will not vanish when c = 0 (and y = y — 0) unless, in addition, 
a = 0 or g77. 
In this case considerations of symmetry would give right results. 
(y.) For an example, take 
y = 30°, y = 60°, 
and the couples will be 
K 
9 7 'lir-yfia^a'^cra' 
pV- A, A' x d 3 
± 
v/3 
3 sin 2a 
the upper sign referring to the a, b shell. 
The sign of the bracket will be -f when a = — ±tt. 
,, ,, ,, I OL - 0 01 ’gTT. 
3? •> >•< 5> ® — Hh 47^"* 
(S.) In confirmation it may be noticed that when y — rr y (he., when the spheres 
are equidistant from the origin), and a is zero or a right angle, the couples are equal 
and opposite. 
