An electromagnetic unit to stop the weft yarn in measuring weft feeders for fluid jet looms, includes a rod movable along its axis under the control of at least one electromagnetic coil so as to insert one of its ends, with slack, into a corresponding hole of the weft feeder drum and engage with that end the weft yarn on the side to stop its unwinding from the drum. Onto the movable end of the rod engaging the weft yarn a bush is mounted for mating with the edge of the drum hole for insertion of the rod end, and the sliding of the bush towards the drum taking place with delay in respect of the movement of the rod towards the drum. IPC D03D.