Third sound in he films on copper substrates

We have performed simultaneous measurements of the speed and attenuation of a third sound wave on a glass and on a copper resonator. The third sound wave on the copper substrate is much more difficult to excite with a thermal method, and we can only see third sound waves on the copper substrate for helium films thicker than 14 atomic layers. The measurements were done to prepare for third sound measurements in ³He films on a copper substrate below 1 mK. A cell has been prepared to use electromechanical excitation of the third sound waves which is suitable for these thicker films, and which avoids the extra heat input of the thermal excitation. With a small vibrating wire the difference in superfluid transition temperature between bulk ³He and ³He films can be determined.