327 
1911-12.] On a Continuous-current Motor-Gyrostat. 
casting carries a cylindrical cap, and in this rests loosely the upper end 
of a stilt pointed at both ends. 
To perform the experiment the gyrostat is placed on the stilts as shown 
in the figure ; it is adjusted so that the stilts are vertical and the axis of 
the gyrostat is horizontal, and it is then let go. The gyrostat balances 
with a to-and-fro motion on the stilts. This experiment, which is one of 
extreme difficulty with an ordinary gyrostat, is performed with the greatest 
ease with this motor-driven instrument. 
Fig. 5. — Motor-Gyrostat on bifilar support. 
In this experiment it should be observed that the gyrostat, when without 
spin, is inclinationally and azimuthally unstable. The spin therefore 
removes two degrees of instability, and an illustration is obtained of the 
general theory according to which an even number of degrees of instability 
are removed by the spin. Thus a result which follows from a consideration 
of the roots of a determinantal equation which determines the periods of 
oscillation of the gyrostat about the equilibrium configuration, is confirmed 
experimentally. 
In fig. 5 is shown the motor-gyrostat performing Lord Kelvin’s bifilar 
experiment. The gyrostat is attached by means of bolts, provided with 
