Conventional and Josephson Noise Thermometry have traditionally been used to measure temperatures in the range below 1 K. The first of these techniques is fast but suffers from accuracy limitations since the spectral density of the Nyquist noise fluctuations of voltage or current must be measured. This requires very precise determination of the system bandwidth. We propose here a modification of the method to eliminate this requirement. A HTS d.c. SQUID maintained at 77 K is magnetically coupled to a copper ring of resistance R ～ 10μΩ and inductance L-100 nH. The temperature of this ring may be varied between 10 K and 200 K since it is mounted in vacuum on a sapphire heat link connected to a variable temperature stage. No electrical connections or bias currents to the copper ring are required in this method. The SQUID bandwidth is much greater than the intrinsic bandwidth of the copper ring so that the total fluctuation amplitude of the noise current in the ring can be measured. At temperature T the rms amplitude of the current noise is related to the circuit parameters by the expression = kBT/L where L is the inductance of the ring. Note that the ring resistance is not involved. The ring inductance may be accurately calculated from its dimensions and is essentially temperature independent. Preliminary results of this novel form of thermometer are presented.
展开▼
