I don't have mine in front of me, but the end of the spring that's attached to the rivet is one part of the circuit, and the wiper that runs along the spring is the other part of the circuit. The difference in the length of the spring between the wiper and the riveted end changes the resistance of the circuit and the brightness of the dash lights.

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.