394 



ELECTRICAL ENGINEERING 



The bulb B has two projections on its sides containing the 

 positive terminals or anodes A and A' which are made of graphite, 

 and two projections on the bottom containing mercury, C is the 

 negative terminal or cathode and S is a third anode used only for 

 starting. The large upper space in the bulb is the cooling chamber 

 in which the mercury vapor, which has been heated by the pas- 

 sage of electricity, is condensed and from which it falls down into 

 the cathode again. 



Load 



FIG. 372. Mercury arc rectifier. 



The two anodes are connected to the terminals of the supply 

 transformer TT' and the load circuit is connected between the 

 cathode C and the junction of the two sustaining coils E and F. 



251. Operation. The operation of the rectifier depends on the 

 fact that current can pass through the tube in one direction only 

 from either of the anodes to the cathode and it can only pass in 

 this direction after an arc has been formed at the cathode in 

 such a direction as to make the mercury negative. In the oppo- 

 site direction the tube is a very good insulator and a difference of 

 potential of thousands of volts would be required to produce a 

 current. The starting arc is produced by impressing a voltage 



