Cavendish’s “electric fluid” model
was not far off considering that electrons would not be discovered until 1897.
Cavendish measured conductivity by how much of a shock it gave him.
Davy measured resistance by how much a conducting wire would heat oil
that it was submerged in, and he measured electromagnetism by how much
iron filings the electromagnet would pick up.
Both Cavendish and Davy discovered that temperature effects
See also in The book of science:
Readings on wikipedia: