OF ARTS AND SCIENCES. 227 



3. By breaking the circuit between mercury and copper in the ma"'- 



netic field, a remarkable change of polarity was observed with 

 the electrometer. 



4. An explanation is offered of the fact noticed by Sir William Thom- 



son, that a greater electro-motive force per unit of length is 

 needed to produce a spark at a short distance than at a long one. 



The subject of this paper was suggested to me by Professor Trow- 

 bridge, and throughout all my work he has kindly given me his advice 

 and help. 



