USE OF THE ELECTROMETER IN WHEATSTONE'S BRIDGE. 355 



when the balance is made. This arrangement is of especial value 

 when the resistances are considerable, for the intensity of the cur- 

 rent in a galvanometer will then be very feeble. 



The use of the electrodynamometer in ordinary conditions will 

 be evidently disadvantageous. The indications of the instrument, 

 being proportional to the square of the current strength, could only 

 be utilised for very weak currents. Moreover, as the deflection is 

 always in the same direction, there is no guide in regulating the 

 resistances. 



These drawbacks* are overcome by placing the fixed coil in 

 the battery circuit, and the movable one in the bridge. The 

 deflection is then, like that of the galvanometer, proportional to 

 the current /, and changes its sign with it. 



In order to get the best conditions of sensitiveness, we must 

 examine the expression 



GS'I/, 



in which S' is the surface of the movable coil; as we have (947) 

 T TT D a'b-ab' 



:E A * = I ~~D~' 



it follows that 



') + r(a + b)\ ]fi (a + a') + a' 



If we suppose equilibrium almost established, we may replace 

 the expressions D and A by their approximate values (948), which 

 gives 



a(a + a'} (a b - ab') 



We might also investigate how the wire of the electrodynamometer 

 should be arranged so as to make the expression GS'I/ a maximum, 

 but this discussion would have no interest, as the sensitiveness of the 

 electrodynamometer is much less than that of the galvanometer. 



961. RESISTANCE OF A GALVANOMETER. The condition of 

 equilibrium of the bridge enables us to determine directly the 

 resistance of a galvanometer, when we cannot dispose of a second 

 galvanometer which enables us to use the first as a mere 



* F. KOHLRAUSCH. Pogg. Ann., Vol. CXLII., p. 427. 1871. 

 A A 2 



