314 



any ordinarily good metallic connexion with binding screws or other- 

 wise ; and join the two electrodes of a galvanic element to their other 

 ends, A, D. Let GPH and KQL he two auxiliary conductors, 

 which, to avoid circumlocutions, I shall call the primary and the 



JG- 



T . "D 



secondary testing-conductors respectively, with their ends applied to 

 the marked points S, T', S f , T. Let P and Q be points in these 

 conductors to which the electrodes of the galvanometer are to be 

 applied. 



It is easily seen, and will be demonstrated below, that if the re- 

 sistances of the testing-conductors be similarly divided in Q and P, 

 and if their ends be in perfect conducting communication with the 

 marked points of the main line to which they are applied; the con- 

 dition that the galvanometer indication may be zero is that the ratio 

 of the resistances of the standard and tested conductors must be the 

 same as that in which the auxiliary conductors are each divided. 

 Further, it is clear that by making the testing-conductors of incom- 

 parably greater resistances than any that can exist in the connexions 

 at S, S', T, T f , which can easily be done if these connexions are 

 moderately good, the error arising from such imperfections as they 

 must present may be made as small as is required*. To de- 



* This method may be readily applied to Siemens's mercury standards (see 

 Phil. Mag. Jan. 1861, or Poggendorffs 'Annalen,' I860, No. 5), by introducing 

 platinum wires through holes in the glass tube near its ends, as electrodes for 

 the testing-conductors, and wires or plates of platinum at the ends, as electrodes 

 for one pole of the battery and for connexion with the conductor to be compared 

 with it, respectively. It will then not be the whole line of mercury from end 

 to end, but the portion of it between the two platinum wires first mentioned, that 

 will be the actual standard. The objection against the use of mercury as a 

 standai'd of resistance, urged by Matthiessen, that the amalgamated copper 



