OF ARTS AND SCIENCES. 193 



VII. 



ON THE INFLUENCE OF MAGNETIC STRESS UPON 

 THE CAPACITY OF AN ELECTRIC CONDENSER. 



By a. S. Kimball. 



Communicated June 10, 1885. 



Recent work in the laboratory of the Worcester Institute of Indus- 

 trial Science has apparently established results of which the following 

 is a brief statement. 



When an electric condenser placed between the poles of a powerful 

 electro-magnet is charged and discharged through a galvanometer^ its 

 capacity as indicated by the swing of the needle is increased by the ex- 

 citement of the magnet, if its lines of force are perpendicular to the 

 plates of the condenser ; but if the condenser plates are parallel to these 

 lines, the capacity is diminished. The percentages of increase in one 

 case and decrease in the other are equal in magnetic fields of the same 

 intensity, and these variations from the normal capacity are greater as 

 the intensity of the field increases. 



Tlie experiments which led to these conclusions were suggested by 

 Maxwell's statements, which indicate a similarity of stress existing in 

 the electro-static and electro-magnetic fields. Maxwell also says that 

 in a field on which electro-static as well as electro-magnetic action is 

 taking place, we must suppose the one stress superposed on the other. 



Several metho^ls of experiment were successively tried, and rejected 

 on account of possible sources of error, before the final disposition of 

 apparatus was reached. 



The first trial was with a h microfarad condenser placed on edge 

 between the poles of an electro-magnet. The cores of the magnet were 

 2 inches in diameter and 13 inches long, and were united Iw a massive 

 soft iron yoke. Each branch was wound with 275 convolutions of wire 

 through which a current of 11 amperes was driven by a dynamo, 

 about ten feet distant. Movable pole pieces were used, and by varying 

 their distance from each other the intensity of the magnetic field could 

 be increased or diminished at will. The current from the dynamo 



VOL. XXI. (n. 8. xin.) 13 



