Ford used a different combination of bi-metallic strip in its flashers to compensate for different numbers of lamps to get a consistent flash rate. When voltage was applied to the flasher, the different metals, steel and copper for example, heated and expanded at different rates and, as the two metals were attached at each end, they bent and the contact at the end to another fixed contact opened. That turned off the circuit, the strip cooled and contact was made again, which heated again - you get the idea. There was a specific electrical resistance that was designed in the bi-metallic strip for two bulbs, turn signals, and four or six, emergency flashers. If you had a trailer wired in, that changed the resistance of the circuit and the flash rate would change. The contacts were made from alloys that reduced arcing. It was simple, no other electrical or electronic component were used, and it worked. The 5 volt constant voltage regulator works on a similar principle.
Jim
