A self-excited dynamo with insufficient residual magnetic field in the metal frame will not be able to produce any current in the rotor, regardless of what speed the rotor spins. This situation can also occur in modern self-excited portable generators, and is resolved for both types of generators in a similar manner, by applying a brief direct current battery charge to the output terminals of the stopped generator. The battery energizes the windings just enough to imprint the residual field, to enable building up the current. This is referred to as "flashing the field".