Polytechnic Association. 961 



or two ohms is sufficient when a single cell is to be measured, but 

 larger when a number of cells are to be measured. 



44. The following formula brings us to a simple and correct rule : 

 Let e — electro-motive force. 

 r =b resistance of the battery. 

 r 1 = a known resistance to be inserted. 

 s — strength of current = tan. of deflection. 

 s x = tan. of deflection when r x is inserted. 

 According to Ohms law, 



e e 



- = s and == s 1 . 



r r + 7 a 



Two equations, involving the two unknown quantities e and r, which, 

 cleared of fractions, become 



e =± rs, 1st equation. 



e = rs 1 + fV, 2d do. 



Eliminate e by substituting rs of the first equation for e of the second 



equation 



rs — rs 1 + ?'V, transferring rs 1 ; 



rs — rs 1 = rV, and dividing by s — s 1 , we have 



s — s 1 

 or the following simple proportion, 



s — s 1 : r 1 : ; s 1 : r. 

 If the galvanometer has resistance, subtract it from the result, and we 

 have the interior resistance of the battery. 



45. Another, method is sometimes employed, in which the resist- 

 ance of the galvanometer must be null, and the wire of an adjustable 

 rheostat so large as not to be essentially heated by the current. 

 e 

 - = s = tangent of deflection, and 



- = tan. of deflection when a resistance is inserted equal to the 



2 



interior resistance. 



Therefore, divide the tangent of deflection by two, and find the degree 

 corresponding to this half tangent ; then interpose rheostat coils to 

 bring the needle to the same degree. 



The resistance thus interposed will be equal to the interior resistance. 



46. The resistance of two cells of equal strength, or two series of 

 two or more cells, each of equal strength, may be obtained by con- 

 [Inst.1 61 



