Youre right, many brands of tractors with that wire coil resistor on the ign/light switch THATS A LIGHT DIMMING RESISTOR.
Many other tractor light/ign switches have TWO RESISTORS on the switch. One (flat wound) is the generators field current limiting resistor for Low or High charge on tractors that used a cutout relay NOT a Voltage Regulator and the other coil of wire/resistor is for light dimming.
The older two cylinder Deeres (720 etc) used a stand alone discrete fixed bathtub style ballast ignition resistor and and a start by pass system so the coil got battery voltage while cranking but reduced (6 volt) when running.
Some of the 12 volt New Generation Deeres (that used a 6 volt coil) had a thermistor/resistor type device on the switch or wired to it (looks like a wire fuse link etc) AND THERE WAS NOOOOOOOOOO START BY PASS NEEDED because the resistance was low for starting but when it warmed up its resistance increased so the coil only saw 6 instead of the closer to 12 volts at starting. I THINK THATS THE TYPE OF DEVICE YOU ARE THINKING OF
Whewwwwwwwwww got all that lol post back an questions