Experimental evidence for mutual friction betweenHe3and superfluidHe4

In the theoretical description of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ flowing through superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ it is generally assumed that below 500 mK the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ can be treated as a "mechanical" vacuum. With the use of dilution refrigerators it is shown in various ways that this assumption is an over-simplification. In one experiment the effects are demonstrated with a double mixing chamber. In another experiment a flow impedance is installed in the dilute outlet tube of a single mixing chamber. Furthermore, the invalidity of the mechanical-vacuum model is demonstrated by experiments in which the properties of these systems are substantially modified by shunting a flow impedance by a superleak.