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January 19, 1860. 



Sir BENJAMIN C. BROD1E, Bart., President, in the Chair. 

 The following communications were read : 



I. "Abstract of a series of Papers and Notes concerning the 

 Electric Discharge through Rarefied Gases and Vapours." 

 By Professor PLUCKER, of Bonn, For. Memb. R.S. Re- 

 ceived December 6, 1859. 



I. Action of the magnet on electric currents transmitted through 

 tubes of any form. 



The action exerted by a magnet on the luminous electric discharge, 

 passing through a tube or any vessel of glass which contains residual 

 traces of any gas or vapour, may be generally explained, if we regard 

 the discharge as a bundle of elementary currents, which, under the 

 influence of the magnet, change their form, as well as their position 

 within the tube, according to the well-known laws of electro-mag- 

 netic action. 



The concentration of the discharge into one free arch only takes 

 place if the arch be allowed to constitute a part of a line of magnetic 

 force. [According to theory, there is no electro-magnetic action at 

 all exerted on any element of a linear electric current which proceeds 

 along such a line.] This condition, for instance, is fulfilled in the 

 case of an exhausted sphere of glass, through which the discharge is 

 sent by means of two small apertures, if the sphere be put on the 

 iron pieces of an electro-magnet in such a way that the two aper- 

 tures coincide with any two points of a line of magnetic force. 



There is another case of electro-magnetic equilibrium, which takes 

 place if the current proceed along an " epibolic curve," i. e. along 

 a curve, falling within the interior surface of the vessel, whose 

 elements, regarded as elements of an electric current, are perpendi- 

 cular to the direction of the electro-magnetic force and impelled by 

 this force towards the surface. An exhausted cylindrical tube, when 

 equatorially placed on the iron pieces of the electro-magnet, presents 

 the simplest instance of this case. All elementary currents are con- 

 centrated by the magnet along one straight line, which, according to 

 the direction of the discharge and to the magnetic polarity, occupies 



