354 



Prof. G. M. Minchin on the Magnetic 



shown in Greenhill's article above referred to), and they take 

 the form of the above equations (14), (15). 



If we were merely to stop the solid in the first process 

 without stopping the liquid, the cyclic motion would cause 

 the liquid to exert a pressure on the solid in the second pro- 

 cess, and the impulse of this pressure would not be zero, but 

 would have to be taken into account in forming the equations 

 of motion. It would be wrong, therefore, to deduce the 

 equations of motion from the impulse applied to the solid 

 alone, as is evident in the analogous case of a solid containing 

 one or more gyrostats. 



. 



XXXV. The Magnetic Field of a Circular Current. 

 By Professor G. M. Minchin, M.A* 



CLERK MAXWELL gives a method of drawing the 

 lines of magnetic force due to a circular current 

 (' Electricity and Magnetism/ Art. 702) by means of a series 

 of circles and a series of parallel lines. The object of the 

 following paper is to show how these curves can be described 

 by a slightly different method, and to exhibit the geometrical 

 connexion of the series of circles. 



Let AQBQ' be the circular current whose sense is indicated 

 by the arrows, the plane of the circle being that of the paper; 



Pig. 1. 



let P be any point in space, PN the perpendicular from P on 

 the plane of the circle, and NAOB the diameter of the circle 

 drawn through 1ST. We shall calculate the vector 'potential of 

 the current at P. 



Draw any ordinate, QQ', of the circle perpendicular to BA; 

 and consider two equal elements of length of the circle, each 

 equal to ds, at Q and Q'. Resolving each of these along and 

 perpendicular to QQ', we see that the latter components are in 

 opposite senses, and hence their vector potentials at P cancel 



* Communicated by the Physical Society : read March 10, 1893, 



