322 Proceedings of the Boyal Society of Edinburgh. [Sess. 
to the free end of the string. Such a gyrostat has serious defects, great 
skill and experience are called for on the part of the experimenter, and 
even then the spin obtained is far from considerable. Again, in the process 
of spinning considerable stresses are applied at the pivots ; as a general rule 
a gyrostat spun in the manner above described deteriorates with each 
successive spin, and is soon rendered useless unless the pivots are repaired 
or replaced. Further, in consequence of friction at the pivots, the spin 
rapidly falls off, and no attempt can be made by its means to do more than 
demonstrate qualitatively the more obvious properties of the gyrostat. 
Experiments of a quantitative nature are quite out of the question. 
It is the object of the present paper to describe a continuous-current 
motor-gyrostat and accessories which have been invented and constructed 
by the authors, the work being carried out in the workshops of the Natural 
Philosophy Department of the University of Glasgow. In designing the 
gyrostat the following points were kept continually in view : — (1) The dis- 
tribution of matter in the flywheel should be such as to make the moment 
of inertia as large as possible for a given mass ; (2) the mass of the non- 
spinning parts should be reduced to a minimum ; (3) the design should be 
such as to allow of the angular velocity being high ; (4) there should be 
effective means of keeping the motor cool ; (5) the gyrostat and accessories 
should be such as to allow of experiments, both qualitative and quantitative, 
being carried out with perfect ease and safety. 
The construction of the motor-gyrostat will be seen from figs. 1 and 2. 
Fig. 1 shows the instrument in section and side elevation. The arma- 
ture-core A, B is made of malleable cast iron or mild steel, and has the 
form of a ring A, B, provided with equally spaced cavities on which are 
wound the armature coils Q. The coils are all wound in the same direction, 
the end of one coil being connected to the beginning of the next, so as to 
form a complete circuit. The junctions between the coils are connected to 
a series of commutator studs J through the medium of clamping nuts K. 
The armature ring is supported centrally from the shaft by means of two 
magnalium discs D, perforated to allow of the armature and field magnets 
being cooled by air circulation. The discs are fitted with gun-metal bushes 
N so as to form good working surfaces on the shaft, upon which they 
rotate. Experiment has shown that this form of bearing is superior to 
ball-bearing races. The magnalium discs are recessed near their outer 
edges to fit tightly into the inner periphery of the armature, and are fixed 
in position by means of countersunk screws, let in flush with the surfaces 
of the discs so that the rotating parts may be as free from projections as 
•possible. The armature ring together with the magnalium discs form a 
