248 



ELECTRICAL MEASUREMENTS 



through which the attracting plate must be moved in order 

 to again bring the cross-hair to its standard position after the 

 application of the P.D. to be measured is S S'. 



The glass jar which forms the case of the instrument is coated 

 with tin foil both inside and outside, and serves as a Leyden 

 jar to hold the auxiliary charge, which is obtained from an 

 electrophorus and is kept constant by the use of a small 

 influence machine called the replenisher. It will be noticed 

 that it is not necessary to know the value of the auxiliary poten- 

 tial; it must, however, remain con- 

 stant throughout the experiment. 

 To test this a small attracted disc 

 electrometer called a gage is pro- 

 vided. As the electrometer is con- 

 structed, both the replenisher and 

 the gage are included within the 

 case. 



The attracted-disc principle is 

 used in secondary electrostatic volt- 



FIG. 136. Simple quadrant electrometer. 



meters intended for high-tension work (see page 258). In such 

 instruments the guard ring is omitted. 



The Quadrant Electrometer. The quadrant electrometer, 

 also the invention of Lord Kelvin, is more sensitive and of much 

 greater practical importance than the absolute instrument just 

 described. Of late years it has been used in investigations con- 

 cerning the energy losses in dielectrics intended for high-voltage 

 insulations. 



The instrument, reduced to its elements, is shown in Fig. 136, 





