At one point in his fabulously varied career, my late father built a "BH loop" plotter for SEI, a UK firm that made ferrites and was based in my town. His machine was a hybrid - a fantastically complex gearbox controlled variacs and resolvers to apply varying H and measure B, while driving an HP X-Y plotter. I have pictures in his archives somewhere, but I vividly remember this lovely machine as 9 year old proto-engineer.