(iii) If terminal MT2 is positive w.r.t. MT1, the triac turns on and the conventional current will flow from MT2 to MT1. If the terminal MT2 is negative w.r.t. MT1, the triac is again turned on but this time the conventional current flows from MT1 to MT2.
The above action of triac reveals that it can act as an a.c. contactor to switch on or off alternating current to a load. The additional advantage of triac is that by adjusting the gate current to a proper value, any portion of both positive and negative half-cycles of a.c. supply can be made to flow through the load. This permits to adjust the transfer of a.c. power from the source to the load.