A method to study speed and attenuation of fourth sound at low frequencies in a long glass capillary

Measurements are reported of speed and attenuation of fourth sound at low frequencies, generated in a well defined geometry, essentially a U-tube, formed by a 117 m long glass capillary (i.d.2r=170+-6μ), wound into a spiral, filled with liquid helium. One of the legs can be moved up and down in a continuously adjustable frequency, in the range of 0.5-3.5Hz, thus imposing a periodic level difference. The level in the fixed leg is observed to remain at rest most of the time, except for special values of the driving frequency, corresponding with standing waves in the capillary. The speed of the wave is calculated from the length of the capillary and the frequency. One can calculate the damping coefficient from the rate of decrease of amplitude, observed after turning off the motor. (Auth.)