Surface tension ofHe4below 0.35 K

The surface tension ($\sigma$) of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ purified to contain 0.4 ± 0.2 parts per ${10}^9$ ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been measured as a function of temperature in the range $0.02<T<\mathrm{}0.35\mathrm{}$ K. It is found that the data agree with Atkins's ripplon theory for the temperature dependence of $\sigma \mathrm{}\left(T\right)\mathrm{}$. The measurements complement earlier work in the temperature range $0.02 \mathrm{K}<T<\mathrm{}{T}_{\lambda }$ in which the surface tension below ∼0.3 K was affected by the natural ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurities.