368 On Measuring Resistances with Wheatstone's Diagram. 



ances, which can be measured only with a great difference in the 

 branches a and d. 



With the usual bridges generally applied in cable-testing, we 

 can measure resistances between t ^q -1,000,000 Siemens' s units, 

 having in the branches both a and d the resistances 10, 100, 

 and 1000 ; and in the comparison-coil, resistances variable from 

 1-10,000 Siemens's units; and it is undeniable that, having to 

 choose the resistance of a galvanometer with one coil only for 

 this kind of bridge, our law must apply to all resistances between 

 100,000 and 1,000,000 Siemens' s units, or to the average resist- 

 ance 550,000. 



To measure 550,000 Siemens' s units, we have to put in equa- 

 tion (2) 



0=10, 



d=1000, 



6 = 5500, 

 and 



c = 550000; 

 and then we have 



# = 1009 Siemens' s units. 



For practical use it will be better to write equation (2) in the 

 following form, 



(a + d\ 



which is identical with the other, because ac—bd is infinitely 

 small. 



If c, the resistance to be measured, is very large in proportion 

 to the largest branch d } we can neglect d against c, and therefore 

 we have approximately 



«g = a + d',» 



or, in this case, 



To raise the magnetic moment of a galvanometer to its maxi- 

 mum, its resistance must be equal to the sum of the two 

 smallest branches. 



[In this case it will be necessary to add a correction to equa- 

 tion (2), which I intend to give in a subsequent article.] 



