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Measurements of both A?B and B?A transitions have been made as a function of magnetic field up to ~100 G at temperatures and pressures close to the polycritical point (PCP). Forms for the specific heat differenceCA-CB=C0(P)-a(P)(Tc-T)/Tc and for the magnetization differenceMA-MB=ßH(Tc-T)/Tc fit the data satisfactorily except very nearTc, where some other mechanism, thought to be a free energy difference due to residual heat flows, tends to stabilize the A phase. The pressure of the PCP, taken to be that at whichC0=0, is found to be 21.22±0.02 bar.

The propagation of 5-, 15-, and 25-MHz sound in superfluid 3He has been studied in zero magnetic field over a wide pressure range. Measurements of the structure and location of the attenuation peaks, velocity changes, and the low-temperature attenuation are described. The degree of strong coupling in the B phase is discussed. Observations of superheating and supercooling near the polycritical point have been extended and comparison is made with theory.

Experimental measurements of the cooling (A?B) and warming (B?A) transitions between3He-A and3He-B are presented for fields of 378 and 5 G. The relationship of these measurements to the existence in zero field of a polycritical point on the line of second-order transitions is then discussed.

Superfluid density of3He has been measured using fourth sound in two superleaks: “confined,” packed powder and “open,” parallel channels. The superleak pore size has little effect on the superfluid density near the melting pressure, but it has surprisingly large effects at lower pressures. The open, parallel channel superleak results show an unexpected pressure dependence of “strong coupling” effects and an unexpectedly small superfluid density change at the B?A transition.

Systematic measurements of the change a – ac of the absorption coefficient for zero sound in3He-A from its value atTc are presented for both 24.08 and 33.53 bar and several frequencies. The3He-A is oriented by means of a magnetic field, so only the case where field and sound directions are parallel give results which can be reliably compared with theory. We find that for 5-MHz zero sound at temperatures well below the collective modes peak there is a substantial anisotropy of the attenuation. We also find at high enough frequency a linear relationship between the sonic frequency and the location on a(1-T/Tc)1/2 scale of the collective modes peak forl·q=0. Comparison of theory and experiment for these data is excellent and suggests that the method may be an accurate one for measuring the temperature dependence of the energy gap in3He-A.

Measurements are given on the bulk nuclear spin polarization in a liquid-solid3He mixture cooled by compressional cooling to below 5 mK in a magnetic field of 54.5 kG. Owing to the low Pauli spin susceptibility of liquid3He, the observed polarization is primarily due to solid3He. A maximum average nuclear polarization of 47% was observed, although the corresponding solid3He polarization is believed to be higher. Our novel detection system, using a dual directional coupler for cw NMR, is a simple and versatile means of working in the awkward frequency range around 180 MHz. We also report transient heating measurements in the3He system which indicate that the internal thermal equilibrium time in bulk solid3He on the3He melting curve appears to be quite short (less than 5 min) at these temperatures. One type of transient measurement is complicated by the dramatic effect of the contribution of the3He nuclear magnetization to the total local magnetic field. This contribution is considered via a simple model.