249 
We shall now treat this problem somewhat more in detail, specia- 
lizing it more than Maxwerr, did. In fact we do not seek for the 
effect of rotatory motions of some wholly unknown kind, but for 
that of the circular currents supposed by AmpPkrr, or, according to 
the conceptions of the theory of electrons, of (negative) elecrons 
circulating round the molecules. For 
the sake of definiteness we shall even 
imagine all electrons to move with 
the same velocity in fixed circular 
paths, all of the same radius. 
Fig. 1 represents this schematically. 
Here OX’, OY’, OZ’ are fixed 
axes, OZ’ being vertical. 
OX, OY, OZ are axes fixed in 
the body. We choose for these the 
principal axes of inertia and denote 
by A, b,C the moments of inertia 
with respect to them. ON is the axis of the coil. 
The center of gravity of the body lies in QV, so that gravity does 
not produce a rotating couple. ; 
Further we shall suppose the terrestrial magnetic field to be 
compensated. 
Both systems of coordinates are of the same kind, so that they 
may be made to coincide by a rotation. 
In the experiment the axis 0)’ was forced to move in a horizontal 
plane, OX, OY, OZ can therefore be brought from the positions 
OX’ OY’ OZ into the actual positions by means of two rotations, 
one about OZ through the angle Y’O)—=y and one about OY 
through the angle Z’OZ7 = 6. 
If there are no molecular currents the kinetic energy 7’ of the 
body is a function of p and 6, viz. 
27 — Ag? sin? 9 + BO? + Cp? eos® B . . . . (Il) 
According to LAGRANGE’s equation we get for the couple tending 
to increase 4: 
d (OT OT 10 ; 
Ol — |= B — dp? (C -— A) sin A cos 0. (2) 
0A 04 dt? 
Here we may remark once for all, that in this experiment the axis 
OY rotates with constant velocity in a horizontal plane about O7’, 
so that is a constant. In order that without an external couple @ 
a stable state may be possible in which OZ and OZ’ coincide, so 
