30 Mr. L. Schwendler on the Galvanometer Resistance 



correction appears necessary in the expression for g. To find 

 this correction, and to inquire whether its influence be really so 

 great as to necessitate its application in practice, is the chief 

 purpose of the present paper. This inquiry is not only of in- 

 terest in the special case of Wheatstone's diagram ; it enters, as 

 insulating covering for coiled wires is always necessary, into 

 all cases where the resistance is to be found for any instrument 

 in any circuit, to produce the maximum magnetic effect. We 

 will consequently conduct the following investigation in such a 

 manner as to make the result of it generally applicable. 



Calling 

 g the unknown resistance which is to produce the maximum 



magnetic effect for a given space, 

 k the external resistance in any circuit, which resistance is always 

 a certain function of the given resistances of the different 

 branches with the exception of g*, 

 q the sectional conducting area of each convolution, 

 A the sectional non-conducting area, consisting of the insulating- 

 covering and the empty space due to each convolution, 

 \ the specific conductivity of the wire, 

 U the number of convolutions necessary for filling the given 



space with wire, 

 we then have 



U 



A 



and 



9 = 



U.B 



thus 



\q '■ 



1+. 

 q 



A and B are two constants for a constant space ; i. e. A is 

 the area of a gross section of the convolutions through one side, 

 and B the length of an average convolution in the given space. 



T> 



— represents, therefore, an electrical resistance which is constant 

 for a given space and constant conductivity, and which may be 



* For instance, in Wheatstone's diagram, 



k= (a+d)(b+c) 

 a+b+c+d 



