220 ALTERNATING CURRENTS 



Even under these conditions, the current becomes zero twice 

 each cycle, so that the arc cannot re-establish itself. By using 

 reactance, Fig. 210, the current is held over each half-cycle, re- 

 sulting in a rippled, uni-directional current wave, Fig. 211 (c). 

 (Also see Chap. XI, page 336, Par. 137(3).) 



The starting anode shown in Fig. 210 is an electrode which 

 sends a current at low voltage through the liquid mercury, 

 causing it to vaporize, and in this way establishes the initial 

 arc. The mercury-arc rectifier used under these conditions has 

 a very high efficiency, as the voltage drop across the tube itself 

 is small compared with the circuit voltage. The tubes, how- 

 ever, are fragile and occasional renewals are necessary. 



(O 



FIG. 211. Rectified current waves. 

 INSTRUMENT TRANSFORMERS 



95. Electrical Measurements at High Voltages. It is not 

 usually practicable to connect instruments or meters directly 

 to high-voltage circuits. Unless the high-voltage circuit is 

 grounded at the instruments, they may be subjected to high- 

 voltage stresses to ground. This makes it dangerous for anyone 

 to come in contact with the switchboard apparatus. Further, 

 instruments become inaccurate when connected directly to high 

 voltage, because of the electrostatic forces which act on the 

 indicating element. Specially designed instruments may be so 

 constructed that they can be connected directly to high-voltage 

 circuits, but these instruments are usually expensive and are not 

 suitable for commercial work. 



