MEASUREMENT OF POTENTIAL DIFFERENCE 251 



The above demonstration is in general sufficient; in it all 

 contact differences of potential have been neglected and it has 

 been assumed that the rates of change of the capacities of the 

 condensers formed by the quadrants G and F and the needle, 

 as the latter turns, are the same and equal to that when the needle 

 is in its zero position. If this is not so there will be an additional 

 moment on the needle, proportional to the difference of the rates 

 of change of the capacities of the two condensers, F and G, and 

 to V 2 ', as this moment is dependent on the voltage applied to 

 the needle it will vary with the uses to which the instrument is 

 put 1 



The Mechanical and Electrical Zeros. When the needle and 

 the two sets of quadrants are at the same potential, there will 

 be no electrical turning moment acting on the needle and it will 

 take up a position due to the suspension alone; this is the me- 

 chanical zero of the instrument. 



When the electrometer is perfectly symmetrical, if the two 

 sets of quadrants are kept at the same potential and voltage is 

 applied between them and the needle, it will be seen from (6) 

 that there should be no deflection from the mechanical zero. How- 

 ever, the slightest departure from symmetry will cause a de- 

 flection and the needle will move to the electrical zero. The 

 difference of the two zeros should be small and in a carefully 

 made instrument they can be made to coincide by adjusting the 

 symmetry of the arrangement. In the instrument shown in 

 Fig. 189, this may be done by tilting the upper quadrants by the 

 use of the vertical screw. The deflection should be read from 

 the electrical zero. 



General Considerations. To obtain a good law of deflection 

 over a long range, it is necessary that the needle be bounded by 

 arcs of circles and straight lines as indicated in Fig. 136. Rais- 

 ing or lowering the needle (or any tilting of the needle) will cause 

 a change in the constant ; for this reason, it is sometimes preferable 

 to have a considerable distance between the upper and lower 

 quadrants. The consequent decrease in the deflecting moment 

 must be compensated by using a more delicate suspension. Even 

 when the greatest care is exercised, the constant of the instru- 

 ment will not be the same for all deflections. 



If the instrument is to be used for alternating-current meas- 



