3RD SOUND IN HE FILMS ON COPPER SUBSTRATES

We have performed simultaneous measurements of the speed and attenuati on of a third sound wave on a glass and on a copper resonator. The thi rd 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 c opper substrate for helium films thicker than 14 atomic layers. The me asurements were done to prepare for third sound measurements in He-3 f ilms on a copper substrate below 1 mK. A cell has been prepared to use electromechanical excitation of the third sound waves which is suitab le for these thicker films, and which avoids the extra heat input of t he thermal excitation. With a small vibrating wire the difference in s uperfluid transition temperature between bulk He-3 and He-3 films can be determined.