Air Bearings
Clock pendulums are normally supported by hanging them from a suspension spring or knife edge. Both solutions work well when carefully designed and constructed but some problems still exist (note: these are minor for most clocks):
The properties of a spring may change with temperature and affect the performance of the clock.
The shape (initial and after wear) of a knife edge affects the performance of the clock.
Energy losses in the supspension lowers the Q of the pendulum.
The location of the "pivot point" of a suspension spring has been a question of debate in horological circles.
Air bearings have the advantage of providing very low friction, no wear and very low dependence on temperature. It is also claimed that they can be more than 100 times stiffer than ball or roller bearings. They are not a practical solution for every day clocks because they are relatively expensive and require a noisy air compressor to function but they may be useful as a test stand for experiments.
Early air bearings were very expensive because they were custom made and required very close tolerances. They were very easy to damage because they became unstable unless air pressure and load were carefully controlled. The advent of porous carbon changed things. Porous carbon air bearings are less fragile - they can survive a complete loss of pressure and you can score them with a knife and they will still work. Porous carbon bearings are available as "off the shelf" design components from New Way Bearings.
Air bearings require high quality air free from oil and water.
Both of the air bearing clocks I have built utilize a MicroSet sensor mounted on an adjustable support to make measurements of period and amplitude using the "Modified Smith Method" as described in HSN 2002-2 (note - the "Modified Smith Method" uses an offset sensor and dark/light interval lengths to measure both period and amplitude). The impulse was provided by Bryan Mumford's E/M controller which is also mounted on an adjustable support.
Links to the two air bearing clocks I have constructed are below: