Magnetic Field due to an Elliptical Current. 107 



ever, to investigate the possibility of transmitting these 

 invisible radiations through opaque bodies. 



The phenomenon of persistent emission of these invisible 

 rays by the uranium compounds lung after any electrical or 

 luminous stimulus has ceased to be applied would seem, 

 therefore, to bear the same relation to the transient emission 

 of .them in the.Crookes tube as the persistent emission of 

 visible light by phosphorescent bodies does to the transient 

 emission of light by fluorescent bodies. Hence the writer 

 ventures to give to the new phenomenon thus independently 

 observed by M. Bec.querel.and by himself the name of hyper- 

 phosphorescence. A hyper-phosphorescent body is one which, 

 after due stimulus, exhibits a persistent emission of invisible 

 rays not included in the hitherto recognized spectrum. 



June 6, 1896. 



VI. On the Magnetic Field due to an Elliptical Current at a 

 point in its plane within it. By J. Viriamu Jones, 

 M.A., B.Sc, F.R.S., Principal and Professor of Physics 

 in the University College of S. Wales and Monmoatltshire, 

 Cardiff*. 



§ 1. TN a communication presented to Section A of the 

 A British Association f at Oxford in 1894, giving 

 an account of measurements made to determine the value 

 of the International Ohm in absolute measure by the method 

 of Lorenz, I referred to a small error consequent on the fact 

 that my standard coil is wound on a cylinder, the section of 

 which at right angles to the generating lines is not a circle 

 but an ellipse of small excentricity. 



In considering the effect of this ellipticity on the value of 

 the resistance calculated from the observations, it must be 

 noted that the ordinary formula implies that the coil is 

 circular. This formula is 



B = Mm, 



where R=the resistance in absolute measure, 



M = the coefficient of mutual induction of the standard 



coil and disk circumference, 

 n = the number of revolutions of the disk per second. 

 But we are primarily concerned with the balance of the 

 electromotive force between the ends of the resistance when 



* Communicated by the Physical Society : read May 22, 1896. 

 t Keport of Electric Standards Committee, Appendix II., Brit. Ass. 

 Report, 1894. ... 



