134 Lord Rayleigh, On the measurement of the electrical [Mar. 10, 



and notwithstanding its smallness is susceptible of accurate deter- 

 mination. Suppose now that the main current traverses also a 

 German silver strip (Proc, Nov. 26, 1883) provided with tongues 

 between which we require to know the resistance. It is evident 

 that by adjustment of c the combination may be made to give the 

 same effect upon the galvanometer as the German silver strip, so 

 that the required result would be readily obtained from the above 

 formula. If c is taken from a resistance-box, we may find the 

 effects, one greater and one less than that of the strip, corresponding 

 to resistances c and c + 1, whence the value that would give 

 exactly the same effect is deduced by interpolation. In order to 

 guard against disturbance from thermo-electricity the readings 

 should be taken by reversal of the battery, and to eliminate the 

 effects of varying current the combination and the strip should be 

 interchanged as rapidly as possible. 



In practice the resistance of the galvanometer could not usually 

 be treated as infinite, and the interpretation of the results is a 

 little more complicated. In the case of the combination it may be 

 shewn that the current through the galvanometer is 



aby 

 g (a + b + c) + b (a + c)' 



By putting a infinite, or otherwise, we see that the corresponding 

 current for the strip is xj (g + x), if x be the required resistance 

 between the tongues. Equating these, we find 



aba 

 g (a + b + c) + be ' 



This method has recently been tested in the Cavendish Laboratory 

 by Messrs Shackle and Ward (see below), and the results appear 

 to shew that even with so moderate a main current as "2 ampere, 

 the sensitiveness is sufficient, the mean of a few readings being 

 probably correct to T oVo- 



Details of the experiment by Messrs Shackle and Ward. 



The arrangement of the apparatus is given in the subjoined 

 diagram. 



The battery used was a Daniell's, an extra resistance of 2 ohms 

 being inserted in the circuit. The battery terminals were con- 

 nected by a reversing key as shewn in the diagram. A, B, C, D 

 are mercury cups with the terminals of the standard ohms a and 

 b firmly pressed into them : B and G are connected by a thick 



