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Liquid nitrogen is palces placed  in the outer dewer to act as the intitial coolant to cool the system down to 77 K. The system has 2 vacuum jackets, the one of on  the outside to prevent heat leaks and one in the inside with a small amount of air allowed in the inner vaccum so that  there is thermal contact between the inner and outer dewars. First add liquid helium 4 to the inner dewar. dewar,  Then using a spectrum analyzer drive the acoustic thermometer at resonance and determine which mode you are driving. Here wired driving.wires  were connected from the computer into type of dial that was labeled with each sound we would measure (1,2, and 4).The dewar is cooled by opening the pressure valve all the way. Data was taken by increasing pressure until $T_{\lambda}$ is reached. System was driven at 10 Volts with a sweep from 1000-3000Hz when looking for first and second sound and from 200-1400Hz for second sound.