GYROSTATS AND GYROSTATIC ACTION—GRAY. 195 
great disadvantage of having no means of maintaining the spin, and 
the continual renewal of the spin is a great nuisance. 
I have here a gyrostat (fig. 1)! in which this drawback has been 
overcome by the simple and effective device of making the flywheel 
itself the rotor of a high-speed continuous-current electric motor. 
The ordinary gramme-ring armature is well adapted for this. It 
gives a wheel of great moment of inertia, or, as I call it, ‘‘spin inertia”’ 
(that is, the matter of the wheel is distributed so as to be on the 
whole as distant from the axis as possible), which can be run at 
high speed for a long time without trouble of any kind from bearings 
or contacts. 
For my first experiments the motor gyrostat is set up, with the 
axis of the flywheel horizontal, in this mounting, which consists, as 
you see, of a fork perched on a pillar. Notite the possible motions, 
the freedoms, I may call them, of the arrangement. The flywheel 
can turn about its axle, the case can turn about the line of the pivots 
which carry it in the fork, and the fork about a vertical axis provided 
in the pillar. These three axles, which we shall number 1, 2, 3, are 
mutually at right angles and meet at the center of gravity of the 
movable system or gyrostat proper. When thus set up the gyrostat 
is said to be freely mounted. 
With the flywheel at rest I push down on one side of the case, and 
immediately turning takes place, as we should expect, about the axis 
2. Pushing down the other side of the case causes the instrument to 
turn about the axis 2 in the opposite direction. I grasp the fork in 
my hands and turn it about the axle 3 in either direction. Nothing 
unexpected happens; the gyrostat turns with the fork, its axis remain- 
ing horizontal throughout. Again, I grasp the pillar in my hands and 
turn it on the table, and you see that the friction of the axle 3 is suffi- 
cient to cause the fork and gyrostat to move round with the pillar. 
As before, the axis of the flywheel remains horizontal. 
My assistant now causes a current of electricity to flow in the coils 
which form part of the flywheel and in the coils which surround the 
soft iron core of the magnet which is stationary within the ring. So 
far you can only tell that the flywheel is turning by the faint hum 
which its motion sets up. But when I repeat the operations which 
I have just performed on the nonrotating gyrostat, the behavior of 
the instrument is quite startlingly different. I push down on one 
side of the case as before; a resisting force is experienced, and the 
gyrostat turns, not visibly about the axle 2, but about 3, the vertical 
axis. So long as I maintain the tilting force so long does the resist- 
ance and this turning about the vertical persist. I withdraw the 
tilting force, and the turning motion ceases. 
1 Figures on plates numbered consecutively. 
