Additional to Dan's comments:
I've seen an edge wound coil where the coil was fixed, and a central spindle
carried a wiping arm. This had two large contact pads each side of the actual
coil strip. Worked well. I have a description of a similar but larger coil
intended for an 80kW tx (bit big even for a 'Californian' kilowatt!) where the
principle was used: it describes the pads as being 20% carbon, 80% silver.
Another one that works well and was used by Collins, Plessey and Marconi that I
know of, has two drums, one insulating and one of metal. A tape or wire winds
from one to the other. It has a high stray capacity to ground from the metal
drum, but if that's the low potential end of the circuit, it doesn't matter.
That has a lot trouble with rolling contacts.
73
Peter G3RZP
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