386 jNIr. I. B. Cooke 07i the Use of a double Circuit in 

 or^ on substitution, 



7' + •VI) 



Let one of the wires of the electromotor be detached from 

 its connexion witli the other wires, and drawn along in con- 

 tact with the secondary wire, thus modifying the relative 

 length, and of course, in the same proportion, the relative 

 resistances of the two courses open to the current, but leaving 

 the sum of the resistances constant. The above equation will 

 become 



G'=F.^. 

 r + w 



If the electromotor be not very weak, and the galvanometer 

 be sensitive, in order to reduce the current passing through 

 the galvanometer to a magnitude within the compass of its 

 indications, the secondary wire will recjuire to be short and 

 stout, making the resistance xv very small, and consequently L 

 very small in comparison with R; F will then vary very slightly 

 by the relative changes thus caused in r and w, and for prac- 

 tical purposes F, as well as r + w, may be considered constant; 

 so that we may take with little error 



, -jo' j G' w' 



G =r — — , and r.p^ = — , 



r + w G w 



or the quantity of electricity passing through the galvanometer 

 wire will vary sensibly as the length of that part of the se- 

 condary wire included between the wires of the electromotor. 

 We have thus, differing slightly from methods previously 

 proposed, a means of graduating the galvanometer. By the 

 use of two cells of Daniell's constant battery in series, this 

 may be accomplished with sufficient accuracy by the above 

 method. One battery wire, being in fixed connexion with 

 one point of the double circuit, the other wire must be ap- 

 proximated to, or elongatetl from, that point, in contact with 

 the secondary wire, until the needle of the galvanometer points 

 to 1°. If the distance of the battery wires be doubled, so as 

 to include a double resistance, the needle ought to point to 

 2°, if trebled to 3°, and so on, the variations in F, whatever 

 be those in G and w, being wholly inappreciable. 



F = G. . If the section and length of the secondary 



wire be properly proportioned to those of the galvanometer 

 wire, it will always be possible, by moving one battery wire 

 backwards and forwards in contact with the secondary wire, 

 while the other battery wire is in fixed connexion, as before 



