Redetermination of the Hyperfine Splitting in the Ground State of Atomic Hydrogen

A redetermination has been made of the zero-field hyperfine splitting of the ground state of atomic hydrogen. A microwave absorption technique was used employing a resonance line with a width of about 3 kc/sec. This width (occasionally as narrow as $\frac{1}{10}$ of the normal Doppler breadth) was obtained through the mechanism of collision reduction of the Doppler effect. The primary relaxation mechanism was electron exchange between hydrogen atoms. The measured result for the hyperfine splitting is $\Delta \nu =1420.40580\mathrm{}\pm 0.00006$ Mc/sec.