How does an electronic caliper work?

How do these electronic calipers work?:

I know they work by measuring the capacitance of the running track, somehow. But how do they use capacitance to measure distances - is it a linear relationship of capacitance to distance, or is there something else going on? These are really accurate - specification of ±0.02mm from 0-100mm, and the resolution is down to 0.01mm. I'm also surprised at how these can meet the specifications at the very low price point - I picked up mine for £8 and tried it against a few common objects I knew the dimensions of, and it checks out.

Answer

Its position to capacitance ratio to frequency ratio to value conversion. The key is using unevenly patterned conductors in proximity of two capacitors. The circuit has slow response, but works remarkably as caliper.