A source of a.c. voltage is coupled through the output winding to the primary winding for inducing a.c. voltage signals in the secondary winding when d.c. control voltage is applied to the input winding and for causing the closed tertiary winding to load the transformer so that little, if any, a.c voltage is induced into the secondary winding in {HEAD ~ the absence} of {TAIL ~ d.c. control voltage} on the input winding 
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A source of a.c . voltage is coupled through the output winding to the primary winding for inducing a.c . voltage signals in the secondary winding when d.c . control voltage is applied to the input winding and for causing the closed tertiary winding to load the transformer so that little , if any , a.c voltage is induced into the secondary winding in { HEAD ~ the absence } of { TAIL ~ d.c . control voltage } on the input winding
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