[Johnson] From the days of non-polarized and non-3-wire plugs.

[Guy Olinger, K2AV]

Part of the business of understanding why Johnson rigs built in the
50's and earlier were built as they were:

Electric wiring started as something called knob and tube. That was a
pair of insulated (usually) wires running through stand off
insulators, usually held down by a nail. Some insulators held both
wires, some just one. Where they went through a wall, they may have
used a porcelain tube which was inserted in a larger drilled hole. The
wire went through the tube.

Which side of the circuit (if either) was the equivalent of a modern
neutral was not known. AC receptacles had two slots, both the same
size, and either one of them might light you up against ground. There
was no way to put a single fuse in the hot side of the AC for
protection. There was no dependable safety ground. It was possible in
a house to have every fuse in the fuse panel on a side of the circuit
closest to ground potential.

Until polarized plugs and three-wire grounded plugs, you were guessing
what wire was what.

When 220 with a grounded neutral center tap started showing up, and
became the standard for NEW installations, designs started showing up
making use of it. We were now in a time of mixed new and old AC
circuitry in use, sometimes mixed in the same house. Both wires could
be at hard potential above ground or either, or neither.

Well-made rigs of the time used transformers to isolate everything
from the AC. If a transformer shorted, possibly grounding a wire to
the transformer case, the only way to keep from putting the case at AC
was to fuse BOTH leads. The fuses had to be matched to insure that
both leads' fuses blew in an internal transformer short.

The experience of all that could go wrong in the old electrical wiring
is what started the National Electrical Code. And even when a new code
provision was enacted, it could be decades before design could count
on the implementation. Then the laws began specifying what could be
manufactured. That hastened compliance to the modern system.

The two fuse plug was a good shot at protection, for its time.

A lot of rig restorers keep them in place.

These days, they would be better converted to a three wire plug and a
single chassis fuse. But then they wouldn't be "original", would they.

73, Guy.