I've updated: http://www.mtaonline.net/~hheffner/HullMotor.pdf
to add: "Regarding Fig. 5, a strategy to improve conductivity and yet retain some of the effective flux is to construct the driver disk using a thin copper disk sandwiched between two iron disks. A further improvement would consist of cutting fine radial groves in the outer portion of the central copper disk in order to keep the radial current tightly confined geometrically. It is possible to construct both the brush disk and active disk in an identical manner, and thus obtain two active disks. The control experiment then consists of replacing the iron disks with copper disks."
Best regards, Horace Heffner http://www.mtaonline.net/~hheffner/
