IRON AND STEEL IN A ROTATING MAGNETIC FIELD. 
717 
tarns of No. 16 wire, one end of the exciting 1 circuit being attached to an insulated 
ring, the other to the magnet. The current is led in by brush contacts. One of the 
bearings of the electro-magnet is in the yoke piece, the other is in a gun-metal 
bracket bolted to the ends of the pole pieces. This bracket carries the insulated 
collecting ring. A third bearing is arranged in a bracket placed between the pole 
pieces to prevent any bending of the extended end of the spindle. The spindles are 
of steel, and are held by set screws in upright gun-metal standards bolted to the bed 
plate. The rigidity of the electro-magnet was ensured by strong bolts passing 
through the yoke piece, by the bracket across the ends of the pole piece, and by 
additional brass brackets of L-section across the sides of the pole pieces. This was 
found to be quite satisfactory, not the slightest movement of the pole pieces being 
detected. The armatures were cylindrical in form, and were made of very thin 
plates strung on a manganese steel spindle 2 - 8 miliims. diameter, and held in place 
by ebonite washers at each end. Some difficulty was experienced in obtaining 
satisfactory sheets of charcoal iron, but finally some exceptionally thin plates, 
*081 millim. in thickness, were obtained from Messrs. Knight and Crowther, who 
very kindly had them specially rolled for the purpose. 
The metal was almost pure iron, containing T4 per cent, of carbon and only traces 
of silicon. Its specific resistance was 1 '2 X 10“ 5 ohm. The plates were insulated 
from each other by a layer of tissue paper. The armature was 2'60 centims. in 
length and consisted of 250 plates. The diameter was 1*75 centims. A second 
armature of hard cold rolled high carbon steel was made with plates T41 millim. in 
thickness, consisting of 147 plates similarly insulated with tissue paper. Its specific 
resistance was 1*5 X 10“ 5 ohm. 
The springs used were of non-magnetic material in the form of a flat helix. To 
avoid disturbances the spring was enclosed in an ebonite box through which the 
armature spindle passed. The armature was supported by the pointed and hardened 
ends of the steel spindle, which rested on hollow coned bearings of hardened steel, 
giving sufficient strength while causing a minimum of friction. 
The magnet was driven by a small leather belt from an electric motor. Owing to 
the construction of the apparatus it was impossible to read the speed directly at 
high speeds, and the value was accordingly calculated from that of the driving motor. 
The magnetising current was read on a Siemens’ electro-dynamometer, which had 
been calibrated from a Kelvin balance. 
In order to ascertain whether the presence of windage or eddy currents in the 
spring attachment or spring or steel spindle produced any appreciable action, a 
preliminary test was performed with a dummy armature in which the iron was 
replaced by ebonite, everything else remaining the same. At the highest speeds 
and with the strongest fields, the effect was inappreciable, and errors from these 
causes were certainly absent. 
When the magnet rotates, the armature revolves with it until the torque exerted 
