PURPOSE: To prevent operation miss or malfunction before it occurs by detecting that the output current of an exciting power source is in the process of increasing or decreasing with two detectors and locking a permanent current switch at ON or OFF through OR of both detection outputs.

CONSTITUTION: Contacts S1 close when the output current Ie for an exciting power source is in the increasing process, i.e., at the linear portion 22 and contacts S2 close when the current Ie is in the decreasing process, i.e., in the linear portion 24. When either S1 or S2 is held closed, the current Ih of a heater coil 3a is locked in the state of ON or OFF. When both S1, S2 are open, they correspond to the linear portion 21, i.e., at the output current Ie=0 of the exciting power source or the linear portion 23, i.e., at the time of current Ie = constant ≠ 0, at which time the current Ih is unlocked and the ON-OFF operations of the heater coil are made possible.