Not “field” as in “field recording,” but “field” as in “magnetic field.”
These are from a chapter on direct-current dynamos in E. Rosenburg’s Electrical Engineering: An Elementary Text-Book. They were made by the General Electric Company, better known today as GE. There are some formulas in there for calculating EMF (a number, not an invisible thing) based on coils and brushes and it’s all a bit beyond me right now. Lovely aesthetic, though. There are also “regulating resistances” made from stamped iron and two-pole switches with magnetic terminals and… great stuff. Very mid-century. Functional beauty.