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The Model A and several other early cars had unregulated generators. Moving the third brush will set the amps to a constant amp output, but if the battery is out of the circuit the voltage can go as high as 40 volts. That's why electronic voltage regulators are such a good thing. With an EVR the voltage is regulated to about 7.2 to 7.5 volts, so even if the charging circuit has a poor connection or blown fuse, the generator will still maintain a safe output and not blow all the lights.