Hmm. "We rub our Tubes with Buck Skin", he says in the same letter. I was trying to work out whether the tube gets a positive or negative charge. I think it depends on what material is being rubbed with what:

https://en.wikipedia.org/wiki/File:Triboelectric-series_EN.s...

The tube is glass, but is the buckskin fur, or slightly furry, or leathery? That would seem to alter the charge it gets.