Electronic correction for air pressure variations
At atmospheric pressure, a pendulum is slightly buoyant to a ‘sea’ of air. This buoyancy causes the pendulum's timing to be a little sensitive to the air's density, and consequently to its pressure. Timing error can be corrected mechanically by putting a small bellows-supported weight on the pendulum. As the air pressure increases, the bellows shrinks and lowers the weight resting on top of it, speeding up the pendulum to compensate for its natural slowdown with increasing pressure. This assumes that the bellows' weight is located in the pendulum's upper half. If located in the lower half, the bellows-supported weight must hang below the bellows instead of sitting on top of it. The effect of the air pressure variations can also be corrected for electronically, using a silicon-based pressure sensor, some electronic circuitry, and an electromagnetic (coil and magnet) pendulum drive. This chapter describes an electronic circuit that corrects for a pendulum's air pressure variations.
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