204 Prof. Gr. M. Minchin on the Magnetic Field 



c and e are wound close together so that there is no appre- 

 ciable time-lag in any current induced in c by e, the small 

 error due to the mutual induction of these coils cancels out. 



Fig. 2. 



1 a 



L- J 



nwn 



Of course a double dynamometer might be used, but then 

 four mercury connexions are needed. It is not unlikely that 

 a method with no mercury contacts can be made available, 

 and greater accuracy could then be attained. 



XVI. Calculation of the Magnetic Field of a Current running in 

 a Cylindrical Coil. By Professor Gr. M. Minchin, M.A* 



LET there be a series of very close circular currents run- 

 ning in the same sense and lying on a right cylinder 

 of radius a whose axis is 00' (fig. 1), and let it be required 

 to find the magnetic potential of this system at any point, P, 

 in space. 



Replace each of these circular currents by its equivalent 

 magnetic shell, which we shall take as a uniform circular 

 plate coinciding with the aperture of the circle. Supposing 

 the currents to circulate in the sense ACB, the upper surface 

 of each plate (as seen in the figure) will be positive and the 

 lower negative. Each circle being touched all round by the 

 one below it, the negative surface of any plate will coincide 

 with the positive of the one next below it ; so that we shall 

 be left with a terminal positive plate, ACB, and a terminal 



* Communicated by the Physical Society : read December 8, 1893, 



