14(5 L'>nl Rayleigh. Constant of Electromagnetic [June 19, 



examples how by the use of the silver voltameter the E.M.F. of any 

 cell can be found without muoh difficulty, and with scarcely any 

 special appliances. 



XI. "Preliminary Note on the Constant of Electromagnetic 

 Rotation of Light in Bisulphide of Carbon." By LORD 

 RAYLEIGH, D.C.L. Received June 18, 1884. 



In connexion with other work upon current measurement by Mrs. 

 Sidgwick and myself, we have endeavoured to determine the value of 

 this constant, so as to decide between the very discrepant results 

 arrived at by Gordon* and by H. Becqnerel.f The method adopted 

 by us was so far similar to that of Gordon that the tube of 

 bisulphide of carbon was placed inside a helix, but the value of the 

 current traversing the helix was determined in a different manner 

 without reference to terrestrial magnetism. 



The light employed was that emitted by sodium. When it is 

 remembered that the effect would vary about two parts per thousand 

 in passiug from one sodium line to the other, the importance of 

 definiteness in this respect will be obvious. 



The number of turns in the helix is 3684, and the insulation was 

 submitted to severe tests. 



In carrying out the measurements the principal difficulty en- 

 countered was from optical disturbance arising from the communica- 

 tion of heat from the helix to the bisulphide. Not only does the 

 mean temperature of the bisulphide rise somewhat rapidly during a 

 series of experiments, but on account of the tendency of the warmer 

 parts to find their way to the top of the tube, the light is sensibly 

 diverted from its proper course. It is believed that by a modification 

 of the apparatus about to be tried, this source of embarrassment will 

 be materially checked. 



The plane of polarisation was determined in some experiments by a 

 Nicol read in two positions, and in others by a double image prism 

 read in four positions. The adjustment of the match between the 

 two parts of the field presented by the half-shade apparatus was 

 facilitated by a device that may be found useful. In addition to the 

 principal helix, the tube was embraced by an auxiliary coil of 

 insulated wire, through which could be led the current from a 

 Leclanche cell. This current was controlled by a reversing key 

 under the hand of the observer, who was thus able to rock the plane 

 of polarisation backwards and forwards through a small angle about 



* " Phil. Trans.," 1877. 

 t " Ann. d. Chim.," 1882. 



