1 86 ELECTROMETRY. 



If <T O and c are the capacities for the position of equilibrium and 

 for the deflection 0, 



(20) 



The capacity changes then in general with the deflection. Never- 

 theless, in case Vj + V 2 = 0, and considering equation (18), we have 



c sm 



The capacity is then virtually constant at any rate, for small 

 deflections. 



)( 814. From formula (18) the deflection of the needle is defined 

 by the product of three factors. The first depends only on the 

 construction of the instrument, the second on the difference of 

 potential of the quadrants, the third on the potential of the needle 

 and on the potentials of the quadrants. 



The second factor is zero when the two pairs of quadrants are at 

 the same potential ; the needle is then stationary, whatever be the 

 charge communicated to it. This would also be the case if the 

 needle were reduced to a circular band ; but from the radii it is only 

 stationary when there is perfect symmetry in respect of the quadrants. 

 This fact is made use of in adjusting the instrument. 



The third factor becomes zero, and the needle is at rest, when- 

 ever the potential of the needle is the mean algebraical potential of 

 the potentials of the quadrants. 



If for a given state of the quadrants the needle is successively 

 raised to equal potentials and opposite signs, the deflections 



are not necessarily of opposite signs, as follows from the preceding 

 observation ; the value of their algebraic sum is 



-(Vi-V 2 ) (V 1 + V 2 )= -(Vj- 



