With an apparatus, designed originally for measuring the osmotic pressure of aHe-4He mixtures, measurements have been made of the isothermal flow of He I I through a narrow sht as a function of the applied pressure head and of the flow rate of He I I and of the superfluid component of aHe-*He mixtures as a function of the applied fountain force. Details of the latter measurements will be published later, this letter deals only with the isothermal flow.
The apparatus consists of a slit, connected on its upper side with a glass capillary, in which the liquid level can be observed, and on its lower end with a copper capillary. The slit, which has a length of 3 cm, is made by melting two gold wires into soft glass. This construction gives a slit width of about 0,2 micron at heliumtemperatures. Both capillaries are connected with the external pumping and filling sYstem and the whole apparatus is immersed in the heliumbath. B y filling the copper capillary with liquid helium to a level, higher than the bath level any vapour pressure between t h a t of the b a t h and the atmospheric pressure can be created in this capillary.
By measuring the increase in height of the liquid level in the glass capillary as a function of the applied pressure head the pressure-velocity relation has been studied at temperatures between 1,25 ยฐ K and the lambda temperature with pressure heads up to 75 cm Hg, corresponding to 7500 cm He.
The pressure differences reached in our experiment are very much higher than those used by W i n k e l c.s. I) and by S w i m and R o h r s c h a c h * ) .
E v e n taking into account the greater length of our slit our pressure gradients are still a factor fifteen higher.
We find, t h a t the relation Ap = B ( T ) . V m still holds up to these high pressures, ~r being the volume, passing through the slit per unit of time. No indication of a saturation velocity has been found, as was expected by S w i m and I{ o h rs c h a c h, when interpreting their last experiments with pressure heads up to 20 cm He. In our experiment m has the value of 3,9.
Comparing our factor B with the one, derived from the experiment of W i n k e 1 c.s. in slits of the order of 1 micron, we find the same temperature dependence of B, viz. B proportional to Q~-4,4 0s being the superfluid density. S w i m and R o h rs c h a c h on the other hand indicate for somewhat wider slits, t h a t V is proportional to Qs for a given pressure difference, which implies, t h a t in their experiment B is proportional to 0s--m, m being the exponent in the relation Ap = B ( T ) . V m and varying between 3 and 4. Comparison of the absolute values of B is difficult, as the dimensions of the apparatus, which are not known exactly, enter in B to a high exponent.
D. H. N. WANSINK K. W. TACONIS