THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1872. 



XXI. On Differential Galvanometers. 

 By Louis Schwendler, Esq."^ 



THERE is one very interesting question connected witli the 

 construction of these instruments which, as far as I know, 

 has not yet been answered, and which is of sufficient practical 

 importance to form the subject of an investigation. 



This question may best be put as follows : — 



A certain battery of given electromotive force and given internal 

 resistance has to supply the tivo coils of any differential galvano- 

 meter with a current ; what must he the resistance of either coil 

 in order to obtain the most delicate reading when measuring a given 

 resistance ?f 



The solution of this problem in its most general form would 

 naturally be extremely intricate, and could not be effected with- 

 out tedious calculation ; but there is one special case where it is 

 comparatively easy to determine the law which connects the re- 

 sistance of the coils with the external resistances to be compared, 

 in order to have the greatest sensitiveness of the instrument. 



Suppose, for instance, that the two coils of a differential gal- 

 vanometer have equal resistances and equal magnetic momenta, 

 and further, that the battery which supplies the two coils with 

 current has an internal resistance sufficiently small to allow of 

 its being neglected against the resistances to be compared. 

 Then, on account of the battery resistance being so small, it 



* From the Journal of the Asiatic Society of Bengal, vol. xli. part 2, 

 18/2. Communicated by the Author. 



t In the Philosophical Magazine of May 186() and January 18()7 I 

 solved a similar question, viz, the proper resistance of the galvanometer to 

 be employed when testing by Wheatstone's balance ; and the result of that 

 investigation has led me to examine the present question. 



Phil, Mag. S. 4. Vol. 44. No. 292. Sept. 1872. M 



