Determination of Low Resistances. 415 



two screws S, S' will raise the springs when required for 

 inserting or removing this wire. 



At points B, C in the standard are soldered two short 

 lengths of wire terminating in binding-screws Q and T. The 

 gauge of the standard wire, which is of copper, and the 

 distance apart of B and C are so chosen that the resistance 

 between B and C is O'Ol ohm at the temperature of the ware 

 at the time of its final adjustment, which should be noted. 

 The method of determining these points is given later on. 



The arrangement is a variation of the Kelvin bridge, with 

 this difference : that in the latter the measurements are made 

 by varying the length between the knife-edges of sliders 

 which press on the standards, of which several are required : 

 whilst in the former only one standard resistance is employed, 

 and the measurements are effected by the alteration of the 

 other resistances in the arrangement. If the whole apparatus 

 had to be bought, the Kelvin bridge would be the cheaper, 

 but the new Addition utilizes the box of coils, which as before- 

 mentioned is sure to be available, and only requires in addition 

 one standard to be made and adjusted. 



The theory of the Kelvin bridge is to be found in the text- 

 books. In Gray's ' Absolute Measurements/ for example, 

 vol. i. p. 359, it is proved that if fig. 2 represents the usual 

 arrangement, and, if R, r, x, y, a, b, s are as marked, the 

 following relationship must hold in order to obtain balance: — 



R x a 



y 



Fig. 2. 



V 



s 



I 



Further, it is shown that when s is small compared with 

 a and b the accuracy of the equation 



R_ x 



r ~~ y 



is not affected by a small want of equality between the ratios 



a , x 

 -r and — . 

 b y 



In addition to the galvanometer and set of coils there is 



