340 



ELEMENTS OF ELECTRICAL ENGINEERING. 



former), it is possible to choose the values of x arid r so that 

 the clock diagram, Fig. 292, is exactly similar to the clock dia- 

 gram, Fig. 291. Under these conditions the voltage e t which 

 acts on the voltmeter bears a fixed ratio to E l at the receiving 

 station and the voltmeter scale is numbered so as to indicate the 

 values of E l directly. 



Fig. 293. 



Westinghouse Static Ground Detector. 

 Fig. 294. 



153. Ground detectors.* The electrostatic ground detector is 

 almost universally used for moderately high voltage alternating- 

 current feeders. The essential features of the electrostatic 

 ground detector are shown in Fig. 293. Two insulated metal 

 plates C and D are connected to the two feeders A and B y 

 and a light metal vane V is suspended between C and D 

 and connected to earth. The grounding of feeder A estab- 

 lishes a connection from A to V, so that the voltage between 

 C and V is zero, whereas the voltage between D and V is 

 large and the vane is attracted by D. The grounding of feeder 

 B in the same way causes the vane to be attracted by C. Fig. 

 294 shows the electrostatic ground detector of the Westing- 

 house Electric & Manufacturing Company. In this instrument, 

 the moving vane is arranged so that it can not be brought into 

 contact with the stationary plates however far it may move, and 

 additional provision is usually made to avoid possible short- 



* The use of incandescent lamps as ground detectors is described on pages 209 and 

 210 of the first volume of this text. 



