CABLE TESTING G90 



The spark-gap method of measuring peak voltages referred 

 to on page 260, is not directly applicable in cable testing, for 

 the observer should be able to follow the variation of the voltage. 



Chubb and Fortescue in their calibration of the sphere spark 

 gap arranged the rotating commutator method (page 627) so 

 that very high voltages, up to 400 kv., could be dealt with in the 

 laboratory. 



The charging current taken by an air condenser is rectified 

 and then measured by a critically damped D'Arsonval galva- 

 nometer. In order that the capacity of the condenser may be 

 accurately calculated, it is provided with guard rings, GR. 

 The two sections of the guard ring are connected together and 

 grounded through a resistance, so that the difference of potential 

 between them and the central or working section will be negligible 

 at all times. The commutator is arranged to short-circuit the 

 galvanometer every alternate half-period and as the brushes are 

 mounted so that they can be displaced, the current may be thrown 

 into the galvanometer for a half-period beginning at any point 

 in the cycle". 



The greatest deflection will be obtained when the brushes are 

 so adjusted that contact begins when the voltage is a positive 

 maximum (+7) and lasts until the negative maximum (-V) is 

 reached. If C is the capacity of the condenser, the total quantity 

 displaced by the unidirectional current will be 2CV. Then, if/ 

 is the frequency, the average current during the contact is 



/ = 4CVf. 



The deflection, D 2 , of the galvanometer, due to a steady cur- 

 rent, 7 2 , taken when the commutator is running, is 



and 



In order to obtain correct results, it was necessary to do away 

 with static troubles by surrounding all wiring, instruments, 

 switches, and resistances with grounded coverings of tin foil or 

 with wire screens. 



The condenser used by Chubb and Fortescue consisted of a cen- 

 tral cylinder with hemispherical ends, diameter 60 cm., lengtl 

 458 cm. This was the high-potential "plate." The grounded 



