Mr. Louis Schwendler on Differential Galvanometers. 263 



tauce of 15 centimetres from the face of a thermopile without 

 its conical reflector, I have often deflected the needle 30°. 



One might suppose that the heat from the source of light 

 would cause currents of air in the shade and make the needle 

 fluctuate ; but I have not met with this difficulty ; and should 

 it arise, it can be removed by placing in front of the condensing- 

 lenses a glass tank containing an aqueous solution of alum. 



The evident superiority of this instrument in simplicity of 

 construction over all preceding devices will at once commend it ; 

 indeed any one in an hour or two can convert his galvanometer 

 into a more convenient and precise instrument for research, and 

 for the illustration of phenomena which have heretofore been 

 presented to students only after considerable trouble on the part 

 of the Professor. Now, however, the whole of the beautiful 

 phenomena of radiant heat, which require a delicate instrument 

 for their evolution, can be exhibited with ease and pleasure before 

 our college-classes. 

 December 31, 18/2. 



XXXIV. On Differential Galvanometers. 



By Louis Schwendler, Esq.* 



Continued from vol. xliv. p. 1/0. 



THE first part of this investigation concluded with the fol- 

 lowing question : — 

 What general condition must be fulfilled in the construction of 

 any differential galvanometer in order to make a simultaneous 

 maximum sensitiveness possible with respect to an alteration of 

 external resistance in either of the differential branches ? 



To answer this question, it will be necessary to remember that 

 the condition of a simultaneous maximum sensitiveuess at or 

 near balance was expressed by three equations^ namely, 



(u;-^)(M/+y)+yi«> + «/+y-^ „ 2(g + w+f) 



p {9 - w )9 l 2 ^9 vy —V {9 + w ) ' 



( Z( /__y ) (w+g)+_J{w + w' +g- 9 >) = 3(g' + «/+/) 

 ~.g %Vg\/gi- 



(II.) 

 (II'.) 



P 



and 



ff'+n>'-p^(^+i^=0, (I.) 



g and g l being the resistances of the two differential coils, w 

 and w' the two resistances at which balance actually arrives, / 



* Communicated by the Author, from the Journal of the Asiatic Society 

 of Bengal, vol. xlii. part 2, 18/3 ; having been read before the Societv 

 March 6", 1872. 



