Just curious what you think about this. The photos at the link are showing large gauge wire used on a vintage 600 VDC rail vehicle. We're talking 1930's electric streetcar technology. Basically 600 VDC connected to a knife switch, then to a rotating manual controller which you partially see in the photo. The rotation of it sets up the series and parallel stages to the traction motors.
We see this weird rubber beading occurring on the outside of the older wire insulation. I'm not sure what that insulation is made of or how old it is. This only seems to occur on the older wires. The newer NFPA compliant wires don't have this symptom.
What is it and what causes it? Any theories? Thanks.