The liquid 3He is essential for good thermal contact. The device is a modification of a standard mutual inductance bridge with counter-wound secondary coils with only one containing the sample and one excitation coil as shown below. The components are selected to optimize thermalization and high sensitivity at ultra-low temperatures. Each electrical lead passes through a “sock” of sintered Ag to thermalize the electrons in the electrical leads.
A mutual inductance bridge is used to read the difference in inductance of the two pick-up coils and illustrated below. The transformer is wound form very fine annealed Ag wire (5μm φ) and no ferrite core is used at these temperatures.
Click on images for details.
- Cell filled with liquid 3He. Cell filled with liquid 3He.
- Mutual inductance bridge. Mutual inductance bridge.
L. Yin et al., Magnetic Susceptibility Measurements at Ultra-Low Temperatures, J. Low Temp. Phys., 158, 710-715 (2010) Read online