318 



sistance S'T can be made absolutely insensible, Wheatstoue's balance 

 leaves nothing to desire, provided the ends of the testing-conductor 

 are applied to marked points on the standard and tested conductors, 

 and the battery electrodes to their outer ends, or to points of them 

 between their outer ends and those marked points. "When, how- 

 ever, as very frequently is the case, S'T may be made small but 

 not absolutely insensible in comparison with the resistances of the 

 standard and tested conductors, the addition of the " secondary test- 

 ing-conductor" becomes valuable, even if it be only arranged to give 



SPT' S'Q 

 a rough approximation to the condition g77yf< ~sp = l *> smce ^ w ^ 



SPT' S'O 

 reduce the error to the fraction -y - ^~ 1, of the small resist- 



OJT 



ance Rt But further, when, as in experiments on short thick bars 

 like those of Mr. Calvert, S'T cannot by any management be got to 

 be small in comparison with TT', the use of the secondary testing- 

 conductor becomes essential, and the most accurate possible fulfilment 



of the condition 



SPT' ^Q 

 S'QT ' SP a 



must be aimed at. This is to be done by dividing the secondary 

 testing-conductor at Q, in very exactly the same ratio as the primary 

 at P, and taking care that the resistances in the connexions S'K, LT 

 are very small in comparison with KQ and QL. 



Part II. Suggestions for carrying out these principles in practice. 

 When high accuracy is not required, the two " testing -conductors" 

 may be made of wires stretched straight in parallel lines, and the 

 connexions for the galvanometer electrodes may be applied to them 

 by means of a slide on a graduated scale as in one of the common 

 forms of Wheatstone's balance, with sliding contact on single test- 

 ing-conductor. This form is very objectionable, however, whether 

 for Wheatstone's balance or the method I now propose : (1) because 



* This of course is equivalent to SPT' : SP : : S'QT : S'Q,, and means that the 

 secondary conductor is to be divided by one galvanometer electrode in the same 

 proportion as the primary is divided by the other. 



t In such cases E will, according to equation (1) above, be nearly equal to 

 S'BCT, but somewhat less. 



