Not your daddies mass driver

From: Ian Woollard

> It's an idea by the late Dr. Forward- a mass driver where the mass
> driver coils pull themselves down the length of a conductive cable
> that powers it,

It's the mass-driver-like coils pulling themselves along the cable part
that I don't get. What would there be, magnetically speaking, for the
coils to grab onto on a continuous, unvarying conductive cable?

Regards,

Mike Combs