1. Triodes for alternating current are Bidirectional and so current can flow through them in either direction.

2. Phase or the halver output to the onefortriggering the counter ci'rcuitsis used and is arranged 'Alternatelyto turn on and cut current in the right hand triodes V2, V4 and Vs while maintaining V3 and V5 cut olf, during one half of each elemental period of the scan.