92 
Proceedings of the Royal Society 
electro-motive force, and exerts electro-motive force when it is 
stopped, like a rotating body having inertia. 
(a) If the friction supposed by Prof. Forbes exists, it would act 
as an accelerating force on the glass, so that if free it would rotate 
faster and faster up to a certain great velocity, and if suspended by 
a fibre, it would rotate till the moment of friction was balanced by 
the moment of tortion of the fibre. 
(J3) If there is no friction the only effects possible would be those 
due, not to the maintenance, but to the starting and stopping of 
the molecular rotation. 
To investigate (a) experimentally we must observe the elongations 
of the oscillation as follows : — 
Make + ve observe three turning points A. B C, break for nearly 
half a complete vibration. Make - ve observe three turning points 
D E F, break again, and make + ve , and so on. Then the result is 
obtained by taking 
I S { A + 2 B + C-(D + 2 E + F) } 
on 
when n represents the number of repetitions of the series of six 
observations. 
To investigate (J3) experimentally we must make and break when 
the mirror is passing the point of equilibrium. 
In Prof. Forbes’s experiments there is a disturbing effect due to 
the ordinary diamagnetic action of the electro-magnet on the tube, 
which, if the tube is not perfectly symmetrised about the axis of the 
fibre, will tend to produce rotation. This force, however, is the 
same whether the current be + or — , provided the position of the 
tube is the same. Hence, if the + and — currents are exactly equal, 
it may be possible to distinguish this effect from the effect sought 
by Prof. Forbes. 
J. Clerk Maxwell,] 
