It's best to use an oil pressure sensitive switch, so if the engine happens to stall when it's cold it won't continue to heat the choke until it's restarted. That's how GM did it when they started using electric-heated chokes in the early 1980's. Ford used a wire from the alternator stator, and actually used AC voltage for the choke heater. Same sort of operation - - - -engine stalls, choke stops heating.
Jerry
