I wonder if that's only about direct current. The higher frequency is AC, the more it conducts on sides of the wire (hence insulated strands). So I wouldn't be

I wonder if that's only about direct current. The higher frequency is AC, the more it conducts on sides of the wire (hence insulated strands). So I wouldn't be surprised by their discovery if we talked about very high frequency AC. But since it all reversed here it might by opposite for AC in their case. UPDATE: paper says something about RF (radio frequency), but I'm not sure I understand what it means, looks more like manufacturing process.