212 ELECTRICAL MEASUREMENTS 



insulating the cable and allowing it to discharge by leakage 

 through its own insulation resistance. At a noted minute and 

 second again charge the cable, noting the elongation D t , which 

 is due to the passage of the quantity Q t through the galvanom- 

 eter to replace that which has leaked out. 



In thus applying the loss of charge method to cables, the as- 

 sumption has been made that the current flow through the in- 

 sulating covering obeys Ohm's law. On account of the non- 

 fulfilment of the assumed conditions, the results are subject 

 to errors; but in many cases of industrial testing the results 

 attained by a definite method of procedure are sufficient. 



It will be noticed that for a given value of -y- the time of dis- 

 charge is proportional to C. In general, the capacity of the cable 

 is great enough to sufficiently prolong the discharge; however, 

 when short lengths are tested it may be necessary to employ an 

 auxiliary condenser. If this be done, a special determination 

 of its effective resistance must be made. The condenser being in 



parallel with the cable, it is customary 

 to compute the resistance of the latter 

 from their combined resistances by 

 the law of divided circuits. From 

 what has been stated it will be seen 

 that this procedure cannot be expect- 

 ed to give results of great accuracy. 

 FIG. 120. Connections for Loss of Charge Method, Using 



Quadrant Electrometer.-The fall of 

 potential may also be obtained by 

 the use of a quadrant electrometer, the needle of which is charged 

 to a high and constant potential. The connections are shown 

 in Fig. 120. 



By closing the switch S the cable is charged and both sets 

 of quadrants brought to the same potential. The electrometer 

 will therefore remain undeflected. When S is opened, the 

 right-hand quadrants are kept at a potential V by the battery, 

 while the potential of the left-hand quadrants gradually falls, 

 as the cable discharges through its own resistance. A deflec- 

 tion appears which is sensibly proportional to v. 



