This goes back to the early days of electricity in the mid 19th C, well before
J J Thomson discovered the electron. (Not sure which one of the million
trillions he found! :-) )

It was not then known exactly what was a flow of electricity and it was thought to
be a stream of some sort. Now, I'm not a student of the Classics, and it is 53
years since I did Latin at school, but I believe that Rheus was Ancient Greek
for a stream, so Rheostat is a stream controller in much the same way
that Thermostat is a temperature controller.

Also from that same period of time, a Rheofore is a length of wire!

73 de Gareth G4SDW