🚀 go-pugleaf

In article <schumach.713227701@convex.convex.com> schumach@convex.com (Richard A. Schumacher) writes:

)Bonus question: suppose that the tether were a coaxial cable, with shield
)and inner conductor shorted together at the far end. If the outer
)shield were superconducting, it would expel magnetic fields, yet the
)Earth's field would still induce a voltage in it (5 kV for a 20 km tether,
)more or less). Could the center conductor then be used as the return line,
)removing the need for plasma contacts at either end? Why or why not?
)

I don't think so.  Regardless of whether the outer cable is superconductiong
or normally conducting, you will still have no net flux change (dB/dt)
through the complete circuit to induce a voltage.

You need a stationary return path to get a net flux change through the
circuit.  With a stationary return path available, there is a net flux
change through the circuit.

This assumes that the earth has a constant magnetic field.  If you have
a spatially varying magnetic field, you could get a flux change through
the circuit, like moving a copper plate through the poles of a magnet.
(Moving a copper plate or loop around in a uniform field should generate 
*no* induced voltages / flux changes / drag, but moving a copper
plate or loop around in a non-uniform field should generate all of the
above.

Actually a superconducting cable would be bad for trying to generate power -
it would keep any flux from entering the cable, preventing any differential
voltage from being generated between the inner and outer conductor even if
there was a time (or space) varying magnetic field.  (Moving through a space
varying electrical field is like being in a time varying magnetic field,
actually).    IE the shielded cable would act like a 'perfect'  shielded
cable, even against slowly varying magnetic fields, if the outer layer
were a superconductor.