Royal Society. 389 



We may as well state that we have used two sets of metallic 

 points — one lent to us by the Professor of Chemistry, the other 

 cut from a piece of indium which we purchased from Messrs. 

 Hopkin and Williams of Hatton Garden. The spectroscope 

 we used is a powerful instrument with four prisms. 



Cavendish Laboratory, Cambridge, 



XLVIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 315.] 



March 30, 1876.— Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



nnUE following papers were read : — 



-*- " An Experiment on Electro-magnetic Rotation." By W. 

 Spottiswoode, M.A., Treas. and V.P.P.S. 



The phenomena of the rotation of movable conductors, carrying 

 currents, about lines of magnetic force are well known. One 

 form of experiment, commonly called the rotating spark, presents, 

 beside the actual rotation, some peculiar features which do not 

 appear to have been noticed m detail. The instrumental arrange- 

 ments consist of a partially exhausted chamber with a platinum 

 point for one terminal, a ring for the other, and the intervening air 

 or other gas for the movable conductor. The chamber is made in 

 the form of a double cylinder, so that a magnet inserted through 

 the ring may reach nearly to the point. The discharge then 

 passes between the point and the ring, and revolves about the 

 magnet according to Ampere's law. 



But beside the rotation, and even when, through weakening of 

 the magnet, rotation does not actually take place, the spark, when 

 carefully observed, is seen to assume a spiral form ; and the spiral 

 is right-handed or left-handed according to both the direction of the 

 current and the magnetic polarity. This effect is particularly 

 noticeable if the magnetic pole be inserted only a short distance 

 beyond the ring. The discharge is then seen to spread itself out 

 sheetwise on the ring in the direction in which rotation would take 

 place. The edge of the sheet is in the form of a helix. 



The object of the following observations is to bring out the 

 character of this phenomenon by making it a principal instead 

 of a secondary feature of the experiment. 



The arrangement here described consisted in using the poles 

 of an electromagnet as the terminals of a discharge from an in- 

 duction-coil, and in observing the effect on the form of the dis- 

 charge caused by exciting the electromagnet. Por this purpose 

 the movable poles were insulated from the main body of the mag- 

 net by interposing a sheet of ebonite thick enough to prevent the 

 passage of the discharge, but not thicker, in order as little as pos- 



