A vehicle includes a differential, a lock and a differential lock controller. The cold-cathode electron source comprises a gate electrode provided on a substrate through an insulating layer and an emitter extending through the insulating layer and the gate electrode and disposed in an opening of the gate. The control circuit is further operable to vary a characteristic of the low current calibrated reference current circuit in a predetermined fashion while comparing another characteristic of the low current calibrated reference current circuit to a predetermined value.