An elementary treatment of the theory of spinning tops and gyroscopic motion, by Harold Crabtree.

DIABOLO

121

At a meeting of the Physical Society of London, in November,
1907, Mr. C. V. Boys exhibited some interesting experiments
with a wooden spool of this description. His spool was pierced
with an axial hole, in which condition the moment of inertia
was greater about the axle than about a transverse axis. The
spool was found easy to spin, and displayed considerable stability.
B A B A
A' B A B
FIG. 71. FIG. 72.
The lecturer then inserted through the hole (Fig. 72), a
small stick of such a length as to make the two moments of
inertia exactly equal, and by attempting to spin the spool
illustrated very clearly its marked instability of rotation about
any axis whatever, and the impossibility of spinning it. On the
same stick being used but pushed slightly from its central
position so as to project rather more at one end than the other,
the spool again was spun without difficulty, and displayed
distinct stability, but at, once began to precess owing to the
gravity couple called into play by the displacement of the centre
of mass. A similar stability of rotation was observed when a
longer stick was inserted, causing the moment of inertia to be
greater about a transverse axis than about the axis of the cone.
It is interesting to notice that in equalising the moments of
inertia the adjustment must be very exact before spinning
becomes impossible. If a torsion wire is applied as a test it is
found that when the difference in periods of oscillation is as
much as 1 in 35, spinning is quite easy. The periods should
not differ more than one per cent.
For several of the above details the author is indebted to the
Proceedings of the above-mentioned meeting, published by
the Society.
133. A problem of stability relating to Schlick's method
of steadying vessels.
A fly-wheel is symmetrically mounted in a spherical case
to which is fixed an axle perpendicular to the axis of the flywheel. The ends of this axle are mounted inside a hollow