On the 'second potential' in electrodynamics

In the strict absence of charge, it is manifestly possible to introduce two potentials in electrodynamics, one of which is well known and can be said to be of magnetic character whilst the other is rather less well known and can be said to be of electric character. Remarkably, both potentials make explicit and seemingly natural appearances in the fundamental description of the angular momentum of freely propagating light. In the present paper, we review and examine the 'second potential' and, in particular, consider its significance in the presence of charge.