as affected by Induced Currents in an Iron Cylinder. 443 



along area I in much the same way as was observed radially that is 

 to say, the intensity of induction diminishes in value and suffers 

 retardation in phase. Table IV gives the values of the induction 

 density and the phase-displacements for periodic times of 360, 90, 

 and 45 seconds. In order to compare the diminution of induction 

 density as one proceeds along the axis of the cylinder towards the 

 centre, the maximum average values of induction density over 

 Sections 1 and I have been plotted in figs. 2, 4, 6. The points are 

 marked x and agree fairly well with the corresponding curves. The 

 phase-displacements are shown in Table IV, and are a maximum for 

 the intermediate force. Comparing the maximum average values of 

 the intensity of induction over Sections 2 and II, we see that the corre- 

 sponding differences are not so great and the phase-displacements are 

 less. The maximum average values of induction density over 

 Sections 3 and III are more nearly equal and the phase-displacements 

 are small. Lastly, comparing the maximum average intensities of 

 induction over areas I, II, III, we see that the diminution is not so 

 great as over the areas 1, 2, 3, and the corresponding phase-displace- 

 ments are less. 



V. We have seen that with a cylinder of 10 inches axial length, 

 the induced currents do not seriously disturb the approximately 

 uniform distribution of induction density over its section, when 

 rotated in a magnetic field with periodic time 360 seconds. This 

 periodic time corresponds to a frequency of 179 periods per second 

 in a cylinder of axial length O'l cm. In ordinary dynamo-electric 

 machines the frequencies usually met with are very much smaller than 

 179, and plates O'l cm. thick are used. The inference is, that in 

 cases 'in which as good an approximation to a pure rotating field as 

 has been obtained in these experiments is met with, no serious devia- 

 tion from uniform distribution exists, provided the plates are insulated 

 from one another. 



VI. Induction motors for a certain class of work are now supplied 

 with solid iron armatures. Suppose such a motor with an armature 

 10 inches diameter and 10 inches long, and with one period per 

 revolution, is placed on a supply system having a frequency of 30 

 periods per second. Fig. 6 refers to a periodic time of 45 seconds, 

 or a frequency of - 022 period per second. To obtain the effects 

 observed at this frequency the armature would have to rotate at 

 frequency 29'978 that is, the effective frequency or "slip," as 

 between the rotating field, which would have to be as uniform as in 

 these experiments, and the armature is 0'073 per cent, of the frequency 

 of supply. To obtain the effects observed at 22'5 second periodic 

 time, the effective frequency would be 0-146 per cent, of the frequency 

 of supply. A slip of 5 per cent, could easily be obtained in practice, 

 and this would correspond to an effective frequency sixty-eight times 



