There is a weak residual magnetic field that persists in the metal frame of the device when it is not operating, which has been imprinted onto the metal by the field windings. The dynamo begins rotating while not connected to an external load. The residual magnetic field induces a very small electrical current into the rotor windings as they begin to rotate. Without an external load attached, this small current is then fully supplied to the field windings, which in combination with the residual field, cause the rotor to produce more current. In this manner, the self-exciting dynamo "builds up" its internal magnetic fields until it reaches its normal operating voltage. When it is able to produce sufficient current to sustain both its internal fields and an external load, it is ready to be used.