Measurement of the velocity of fourth sound in helium II under pressure

Using the resonance method, the velocity of fourth sound has been measured at temperatures between 1.3 K and the lambda line and at pressures between saturated vapour pressure and the melting curve in helium-filled small pores of packed powders. From these measurements, the superfluid fraction rhosub(s)/rho was calculated. Near the lambda line the critical behaviour of rhosub(s)/rho was analysed and a universal value 0.670 +- 0.015 of the critical exponent zeta was found in agreement with previous measurements. The fourth sound technique was utilised to determine the melting curve and the lambda line of 4He. In various powder samples with narrow capillaries depressions of the inset lines for superfluidity, which were parallel to the lambda line, have been observed. Wherever possible, comparison was made with existing data. (author)