Precision Measurement of the Three23PJHelium Fine Structure Intervals

The three $2^{3}P$ fine structure intervals of ${}^4\mathrm{He}$ are measured at an improved accuracy that is sufficient to test two-electron QED theory and to determine the fine structure constant $\alpha$ to 14 parts in ${10}^9$. The more accurate determination of $\alpha$, to a precision higher than attained with the quantum Hall and Josephson effects, awaits the reconciliation of two inconsistent theoretical calculations now being compared term by term. A low pressure helium discharge presents experimental uncertainties quite different than for earlier measurements and allows direct measurements of light pressure shifts.