1876.] On the Residual Charge of the Ley den Jar. 407 



Transposing, squaring, and adding, and putting p for the radius of cur- 

 vature, we obtain 



C 2 R 2 sin 2 i=r 2 : p 2 , or p= t : CE sin i, 



which is constant. The curve is therefore a helix. Also the radius of 

 curvature of the projection of the curve on a plane perpendicular to the 

 axis will be p sin 2 i, viz. =7 sin i : CR. 



" The value of r depends doubtless on the nature and pressure of the 

 gas, and perhaps also on the current ; but it must be the same for equal 

 values of of opposite signs. Hence the handedness of the helix will 

 be reversed by reversing either the current or the magnetic polarity. If 

 the left-hand magnetic pole be north (i. e. austral, or north-pointing), 

 and the left-hand terminal positive, the helix will be right-handed." 



The general nature of the phenomenon may therefore now be described 

 as follows : — " First, we have the bright spark of no sensible duration 

 which strikes nearly in a straight line between the terminals. This opens 

 a path for a continuous discharge, which being nearly in a condition of 

 equilibrium, though an unstable one, remains a short time without 

 much change of place. Then it moves rapidly to its position of equili- 

 brium, the surface which is its locus forming the sheet. Then it remains 

 in its position of equilibrium during the greater part of the discharge, 

 approaching the axis again as the discharge falls, so that its equilibrium 

 position is not so far from the axis. Thus we see two bright curves cor- 

 responding to the two positions of approximate rest united by a less 

 bright sheet, the first curve being nearly a straight line, and the second 

 nearly a helix traced on a cylinder of which the former line is a generating 

 line. 



" It was noticed that the sheet projected a little beyond the helix. 

 This may be explained by considering that at first the discharge is more 

 powerful than can be maintained, so that the curve reaches a little beyond 

 the distance that can be maintained." 



The appearance of the discharge when viewed in a revolving mirror 

 (except the projection beyond the sheet, the illumination of which was 

 too feeble to be observed) confirmed the above remarks. 



II. u The Residual Charge of the Ley den Jar/" By J". Hopiunson, 

 M.A., D.Sc. Communicated by Prof. Sir William Thomson, 

 F.R.S. 



(Abstract.) 



1. If it be assumed that a dielectric under electric induction has every 

 element of volume of its substance in an electropolar state, and also that 

 dielectrics have a property analogous to coercive force in magnetism 

 whereby time is requisite for the development or decay of this electro- 

 polar state, an explanation of the residual charge of the Leyden jar 



