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CURRENT RECTIFICATION 279
(16) Remember that lugs A and Al are charged with voltage from the secondary circuits of the main reactance, usually located at the bottom of the rectifier. Consider, too, that A and Al are connected to opposite sides of the secondary. In other words, A represents one side of the a. c. supply and Al the other. Remember also that voltage is supplied, through the vapor, between A and Al and lug B.
(17) Referring to Figure 74, note that C of Figure 73 is wired in series with a "starter anode resistor" which limits the current that can flow through it, and hence reduces C to a state of practical unimportance except as one of the electrodes for the starting spark. The remainder of the action of this rectifier lies between A, Al and B.
Assuming the a.c. line entering Figure 74 at the top to be of 110 volts, there is a drop of 55 volts across each of the two main reactance coils connected in series across that line. The wire that runs left and downward from the center point between those coils is then negative by 55 volts with respect to whichever of the two wires is positive at any given moment. This center-tap wire may be traced through the arc itself (labeled "lamp" in the drawing) and through a protective relay to B of the rectifier.
B is in contact with a pool of mercury inside the rectifier tube. The starting spark has heated some of the surface of this mercury and caused it to emit electrons. Those electrons are attracted to either A or A1 — whichever of the two is at the moment 55 volts positive with reference to B, and form a conducting path from B that anode. On the next cycle, electrons are repelled from that contact (which has become 55 volts negative with respect to B) and attracted to the opposite side extension of the rectifier. By virtue of this action a continuous current flows from I] through the rectifier, alternately leaving the rectifier at A and at' A1. This direct current flows through the projection arc, which is in series with B.
One technical correction must be applied to the above. The difference in potential between B and A and A1 actu