The low-field proton gyromagnetic ratio γ′p experiment at the ETL

The low-field proton gyromagnetic ratio γ′p(low) experiment at the Electrotechnical Laboratory (ETL) has been continuing with the construction of a new nonmagnetic building, precision solenoid, and dimensional measuring apparatus. A tentative value of γ′p(low)ETL86 and its uncertainty are given: γ′p(low)ETL86 = 2.6 751 289(59) × 10⁸ s⁻¹T⁻¹ETL (2.2 ppm). The sources of uncertainty have been carefully investigated. The irregularity of the solenoid diameter and the resistance calibration mainly caused the large uncertainty. In order to reduce the uncertainties some improvements have been done or are planned.