Why is the capacitance measured for an air-dielectric with the 16452A unstable?

If the capacitance measured for an air-dielectric is unstable, check whether the 16452A electrodes are sufficiently clean or not. Verify that the measured value is within the range given in table 1-1 of the 16452A Operation and Service Manual. Also refer to Note on page 3-12 of the manual for the details of operational check procedure.