In Britain, rural areas wich were electrified in the later years went straight to AC, but those districts which got electricity early on were wired with 3-wire DC systems.
These were gradually converted to AC over the years, but the central districts of some older towns still had DC mains into the early 1960s.
AC/DC radios and TVs of the time used a cheap half-wave rectifier with one side of the line connected directly to chassis. The tube filaments and dial lamps would be series-wired and connected straight across the incoming AC.
With the reversible 2-pin 5A plugs in common use for radios, plugging in the wrong way to a DC outlet would result in the dial and tube filaments glowing fine, but no B+ supply and thus no sound (or picture).
Where a 3-pin polarized plug was used, those living in homes fed from the negative "outer" would need to wire their radios so that the chassis would be live. With non-identified twin cord, it would be trial-and-error to get the connections the right way, or for color-coded cord they would need to connect the red to neutral and black to live (i.e. the opposite way to "normal").
The problem arose then of course, that the plug would need to be rewired for the set to work in a house wired on the other side of the supply!
