CONVERTERS 395 



between the two mercury terminals C and S through the starting 

 resistance r s and tipping the tube until the mercury forms a bridge 

 and closes the circuit. Current then passes and when the cir- 

 cuit is opened by raising the tube to the vertical position an arc 

 is formed and the cathode is said to be excited. If at this instant 

 either of the anodes is at a higher potential than the cathode, 

 current will flow from it and will continue to flow so long as the 

 difference of potential is greater than the counter e.m.f. of the 

 rectifier. When the terminal T of the supply transformer is 

 positive, current flows from it to A through the tube C, through 

 the load circuit and through the reactance coil F to the terminal T". 

 When the voltage reverses and T becomes negative, T f becomes 

 positive and current flows from it to A f through the tube to C 

 and through the coil E to the terminal T. 



If there was no drop of voltage in the rectifier the current 

 wave would be of the same shape as the voltage wave and in the 

 load circuit it would vary from zero to a positive maximum as 

 shown in Fig. 373. There is, however, a drop of voltage of from 



FIG. 373. Rectified alternating current. 



14 volts to 25 volts in the commercial rectifier, which remains ap- 

 proximately constant independent of the load, and if the sustain- 

 ing coils were left out of the circuit the current through the bulb 

 would drop to zero as soon as the potential of T had fallen below 

 the counter e.m.f. of the converter and load circuit and would re- 

 main at zero until the potential of T' rose to a value greater than 

 this counter e.m.f. In the meantime the cathode would have 

 lost its excitation and the tube would have to be tipped again be- 

 fore any current could flow. To prevent the current in the tube 

 from falling to zero and so to insure continuous operation the 

 reactance coils E and F are introduced. Their action is as fol- 

 lows: While current is flowing from T energy is stored in the 

 magnetic field of the coil F and when the potential of T becomes 

 too low to force the current against the counter e.m.f. of the con- 

 verter the sustaining coil discharges its stored energy and main- 

 tains the current' until the potential of T' rises and current flows 

 from it to the load. The effect of the sustaining coils is there- 



