An ultrahigh-sensitivity facility for electrical resistivity experiments in a liquid helium environment

A measurement facility is described which provides direct digital output of the electrical resistance of a metal sample, immersed in He II at 1.7K, with an uncertainty of less than 8*10 -10 Omega and a linearity of +or-0.005%. When it is used to measure the concentration of thermally induced (quenched-in) vacancy-type crystal lattice defects in copper and aluminium the equivalent concentration sensitivity is about 10 -8 . The system uses a microprocessor-cased control and calculation program stored in a non-volatile memory for ease of operation when the equipment is switched on or off.