MEASUREMENT OF POTENTIAL DIFFERENCE 253 



the stiff wire supporting the needle, the intent being to protect 

 the movable system from extraneous electrostatic attraction. 

 The whole tube was supported from above, and carried down 

 through the circular opening at the center of the quadrants, being 

 cut away at the sides to allow free motion of the needle. This 

 construction resulted in the peculiar law of deflection shown in 

 Fig. 137. 



Electrostatic Voltmeters. On the supposition that the law of 

 the quadrant electrometer is that previously deduced, if the 

 needle and one set of quadrants are connected together, V = 

 and 



D = Kd\ 



Or, if the P'.D. is rapidly alternating, 



. 1 



As one pair of quadrants produces no effect, it may be omitted 

 in an instrument designed primarily for voltage measurements. 



As the deflections are proportional to the mean square value 

 of the P.D., electrostatic voltmeters are particularly applicable 

 to the measurement of alternating potential differences. 



These instruments absorb no power and at ordinary frequencies 

 produce no disturbance of the potential difference to which they 

 are applied. Their action is not complicated by inductance 

 effects so there are no frequency or wave-form errors. They 

 have no self-heating errors and are uninfluenced by local mag- 

 netic fields. On the other hand, at low voltages the forces 

 to be dealt with are very small and consequently the instruments 

 are much more delicate than those based on the electrodynamo- 

 meter principle. When they are used on direct-current circuits, 

 the effect of contact differences of potential must be eliminated by 

 reversals. 



Ayrton, Mather and others have developed the electrostatic 

 voltmeter so that it has become an instrument of great value in 

 laboratory work. Fig. 138 shows one of Ayrton and Mather's 

 instruments for low voltages, up to 16 volts. 



This instrument is intended to be read by one of the mirror 

 and scale methods. The suspended system consists of a light 



