54 



47T\Sr J r=a 



Mr. E. C. Eimington on a Method of 



a 



cos ^ 



COS" 







cos 



-f COS 



-1 



COS 



COS 





4tt V ^ A_„ 4a7r" 2 



cos 



9 ^ 9 ^ 



cos 2 ^-— cos^ 



+ COS 



-1 



COS 



COS 



e 



+ - 



Wlien we introduce a and 6 instead of ~- and ~, Thom- 

 son's formulae for the density reappear. It is to be noticed, 

 however, that in this paragraph these formulae have been 

 proved without making use of the theory of electrical images. 



IV. On a Modification of a Method of Maxwell's for 

 Measuring the Coefficient of Self-induction. By E. C. 

 Eimington*. 



IN l Electricity and Magnetism/ § 778, vol. ii., Maxwell 

 gives a method of comparing the coefficient of self- 

 induction of a coil with the capacity of a condenser, the con- 

 nexions for which are practically 

 as follows : — D is the resistance, 

 possessing self-induction ; the 

 armatures of a condenser, of 

 capacity K, are connected to 

 the ends of the arm B. There 

 are keys in the battery and gal- 

 vanometer-circuits. An ordi- 

 nary balance is obtained by 

 depressing the battery-key be- 

 fore the galvanometer-key ; then 

 AC = BD. The resistances B and 

 D are next adjusted so that no 

 momentary deflection shall exist 

 on the galvanometer when its circuit is made before that 

 of the battery. Then L = KBD. 



This method is very inconvenient, as it necessitates a double 

 adjustment. One of the resistances, generally C, has to be 

 adjusted so as to give an ordinary balance ; then, in order to 

 obtain no deflection when the galvanometer-circuit is closed, 



* Communicated by the Physical Society : read May 14, 1887, 



