Direct Measurement of Mean Lives of Atomic States

A method is developed for measuring the mean lives of atomic states by means of the thermal motion of excited atoms in two forms of resonance lamps. The methods are used to determine the mean life of the cadmium $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state for which state the more accurate method gives $T=2.5\mathrm{}\times {10}^{-6\mathrm{}}$ sec. with a probable error which is difficult to determine but can hardly be in excess of 10 percent.