The atom nucleus was discovered by counting how many alpha particles bounced off the nucleus of a piece of gold foil and how many simply passed through it. The bounce rate was incredibly low and therefore the nucleus must have been orders of magnitude smaller than the gap between the next nucleus.
Personally I think you could tell a GE engineer in 1912 that their company would be building a radiation-powered electrical plant and they'd be keen to find out how, not baffled at the very concept. No way of knowing for sure, of course. Just that, this was an era when radioactivity and (what became) nuclear research was very much something the educated were reading about in their monthly magazines.