719 
IRON AND STEEL IN A ROTATING MAGNETIC FIELD. 
lines of force which pass through them, which currents will flow along the sides 
of the spindles and across the ends. The latter portion will be parallel to the lines 
of force of the field, and hence wiil produce a magnetic field at right angles to this, 
causing a slight distortion, but not altering the total magnetic flux between the pole 
pieces and through the armature. Since the spindles were some distance from the 
pole pieces, and since no perceptible heating was produced, it may be assumed that 
no distortion was produced of sufficient magnitude to produce an appreciable change 
in the value of the hysteresis in different parts of the armature. 
The eddy currents in the armature plates may be divided into two portions, 
(1) those due to the lines of force which pass through the plates parallel to their 
planes; (2) those which are produced by the fringe of lines of force at the ends 
of the pole pieces, passing into the armature at the ends in a direction perpendicular 
to the plane of the plates. The first set affect all the plates equally, and can be 
calculated with fair accuracy, but the second set only affect the end plates and 
are not easy to calculate. The distribution of these latter lines of force and currents 
are shown in fig. 1. It is clear that these lines of force will not penetrate far into 
the armature, owing to the numerous small air gaps between the plates, and hence 
the currents will be limited to only a few plates at each end. 
In order to determine the magnitude of this error, an armature was prepared, 
consisting of only 20 steel plates, insulated with paper as before, and 1’7 centim. 
diameter (fig. 2). It was placed in the middle of the polar area. If the effect of the 
eddy currents in the end plates were appreciable, it would be proportionally much 
increased in this armature, and, at a high increase of speed, there should be an 
appreciable increase in the deflexion. (It will subsequently be shown that the 
hysteresis in the steel is independent of the speech) On experiment, it was found 
that there was no normal increase in the deflexion. It may therefore be concluded 
that these end plate eddy currents may be altogether neglected. The test was made 
with various exciting currents, from a low induction up to the maximum obtainable. 
