It is shown that film critical current, defined as the maximum current for which a film shows no resistance, can be strongly increased in one direction and strongly decreased in the other by the field from two current carrying conductors placed parallel to the film edges.
The effect is shown to be due to mutual cancellation of the magnetic fields normal to the film created by the wire currents and the film current. It appears to make a flat film superconductive rectifier possible.
It is proved that the symmetrical increase of critical current due to the proximity of a superconducting ``shield'' plane is due to a similar mechanism, and that fields normal to the film surface are largely responsible for the broad current induced transition of flat ``unshielded'' films.
It is also shown that with mutually opposed wire currents the variation of critical current with wire current exhibits a significant hysteresis. This effect may have applications to digital storage.