I recently bought an old stopwatch from the USSR, I bought it on eBay and it said it was missing parts. I can't figure out what they are. I figured out how to reassemble, but it doesn't tick. There doesn't seem to be a mainspring and I don't know where one would be. Does anyone know how stopwatches work and how they tick without mainspring? There is this pendulum which makes everything move when I ticks. How does it swing, what force is pushing? Thanks for any help. Photo: