MEASUREMENT OF POTENTIAL DIFFERENCE 239 



is increased; then if R M and R v be the resistances of the multiplier 

 and of the voltmeter respectively, 



P.D. - reading times 



When the range is very greatly extended, to several thousand 

 volts, the multiplier is subdivided and mounted in a number of 

 boxes, thus reducing the voltage drop between neighboring wires 

 and rendering it easier to insulate them. Also, capacity effects 

 which might be serious in alternating-current work are much 

 reduced, for high-range multipliers may be subject to errors due 

 to distributed capacity and capacity to ground. 



The condensation of moisture on the resistors of very high- 

 range multipliers, when they are used in air, frequently gives 

 rise to burnouts. These may be obviated by immersing the 

 multiplier in transformer oil. Multipliers often contain soft 

 rubber insulation, this must be removed before the immersion. 



Electrodynamometer Voltmeters. These instruments are 

 primarily designed for the measurement of alternating potential 

 differences. The indicating portion is a comparatively delicate 

 electrodynamometer with a pivoted movable coil. 



The current through an alternating current voltmeter is given by 



v 



where R and L are respectively the total resistance and inductance 

 of the voltmeter and o> is 2irf. At the ordinary commercial 

 frequencies, the indications must be practically independent of 

 the frequency but as L can never be zero, the resistance 

 must be made so high that the reactance can be neglected 

 in comparison with it. The instrument may then be used for 

 both direct and alternating potential differences. This is a 

 convenience, for it may then be calibrated with direct cur- 

 rents, using reversals. 



As the electrodynamometer is a comparatively insensitive in- 

 strument, considerable current is required and it is not possible, 

 while retaining the characteristics of portability and solidity 

 of construction necessary in order that the instrument may have a 

 long life and maintain its accuracy under the trying conditions 

 of everyday work, to give this form of voltmeter as high a re- 



