368 



Prof. E. Wilson. 



[May 28, 



approximately as in these experiments. Bringing to bear the results 

 of Professor Baily's experiments* on the dissipation of energy by 

 magnetic hysteresis in rotating magnetic fields and the results arrived 

 at in this paper, I am still unable to account for the observed loss on 

 the assumption that the plates are insulated from one another. In 

 dynamo-electric machines, where approximate uniform rotating fields 

 are met with, the construction does not lend itself to such perfect condi- 

 tions as, for instance, in the case of alternate current transformer cores. 

 It is doubtful if the lines of force are always confined to the plate. 

 If one considers the section of an armature core in which ventilating 

 spaces are provided, it is highly probable that at the ends of the core 

 and at each ventilating space there is a considerable axial component, 

 which would at once give rise to a serious increase in the dissipation of 

 energy by induced currents. The teeth are sometimes filed out after 

 placing the plates in position, and this may lead to considerable con- 

 ductivity normal to the plane of the plate. Then, again, the plates 

 are not always insulated from the shaft or spider which supports 

 them. In multipolar dynamos it is questionable if one ever meets 

 with a fair approximation to a uniform rotating field. 



Longitudinal Variation of Intensity of Magnetic Induction. 



In the previous paper it was stated that the intensity of magnetic 

 induction diminishes in value and suffers retardation in phase as one 

 proceeds from either end of the cylinder along its longitudinal axis 

 towards the centre, in much the same way as was observed radially in 

 a plane at right-angles to the longitudinal axis and midway between 

 the ends of the cylinder. This statement rested upon a comparison of 

 the electromotive force of coil I with that of coil 1. In the present 

 experiments an opportunity to examine this further was given, and 

 coil 3' (fig. 1) was wound round an area 22*6 sq. cm. at the end of the 

 cylinder. The maximum values of the induction density over this area 

 are given in Table I, so that a comparison can be made with the maxi- 

 mum value of the intensity of induction at the centre (coil 1). The 

 phase-displacements are also given. The results further confirm the 

 statement made in the previous paper. 



Conclusion. 



In my former paper I omitted to thank Professor Capper for the loan 

 of the worm and worm-wheel used in these experiments. I have pleasure 

 in doing so now. I have also pleasure in acknowledging the help I have 

 received from Mr. F. S. Eobertson, and Messrs. M. S. Duffitt and 

 R. M. Wartze. I wish to mention the intelligent interest taken by 

 Mr. Duffitt in the theoretical part of the work, and the patience and 

 care Mr. Wartze has given to the working out of results. Mr. T. Jones 

 has again rendered valuable assistance. 



* See 'Phil. Trans.,' A, vol. 187 (1896), p. 723. 



