Time-Dependent Frequency Shifts in the Hydrogen Maser

A long-term frequency shift, with respect to TAI, has been observed in each of two hydrogen masers by direct frequency comparisons between them and primary cesium standards. The shift is attributed to an increase in the wall shift of the storage bulb in the maser. Depending on the storage bulb used, this increase was found to be between 3 × 10-13 and 8 × 10-13 per year. Possible reasons for the observed shift are discussed.