How can residual voltage be described?

Residual voltage refers to the small amount of voltage that remains in a generator's output even when there is no field current supplied to the generator. This phenomenon occurs due to the residual magnetism that is retained in the generator's magnetic core. When the generator is operated, it builds up a magnetic field, and after the field current is removed, some magnetism remains, allowing for the generation of a small voltage.

In this context, the description captures the essence of how generators operate and how they can still produce voltage due to the residual magnetism left in the system. This residual voltage is crucial for starting the generator again, as it helps in re-establishing the magnetic field when field current is re-applied.

Other choices do not accurately explain residual voltage in the context of electrical systems. For example, voltage without any magnetic influence does not incorporate the essence of how generators work; voltage from a disconnect battery or for testing the integrity of an electrical system has different implications that do not pertain directly to the concept of residual voltage.