Wall Shift of FEP Teflon in the Hydrogen Maser

Abstract

The wall shift of the atomic hydrogen maser has been determined by a series of 18 measurements on a variety of storage bulbs coated with FEP Teflon. The result is −24.3 mHz for a 15.25 cm (6 in.) bulb at 30°C. This is about 25% smaller than the previous values. It is most likely that a change in the properties of the Teflon caused the difference, which is far larger than possible measurement errors. The systematic correction due to the entrance stem has been estimated using several independent models. The fractional variation of the wall shift with temperature has been measured in the range 27 to 56°C and found to be (−11.4±0.4)×10⁻³/deg at 35°C. It is suggested that care be taken in applying the value of the wall shift found by one laboratory to bulbs prepared by different techniques or with different samples of Teflon.