A Secondary Standard of Current Based on a Nuclear Resonance Spin Oscillator

A description is given of the use of a nuclear spin oscillator as a transducer, connecting a magnetic field (10 g) to a frequency (42 kc/s). It is shown that for this function the spin oscillator is to be preferred to the nuclear maser. The system offers the advantage of permitting work in an ordinary laboratory with a background of parasitic field of some 1 mg; this insensibility to fluctuations is very convenient for the preliminary adjustment. For an accurate determination of current by means of a standard solenoid the stray field must be compensated statically and dynamically; for this purpose a feedback arrangement using a galvanometer-photocell amplifier is suggested. Calculations and experiments on an efficient mu-metal screen are presented. But this last arrangement, for absolute measurement of I, would require too large a screen when using a solenoid that shows a high-leakage field. The screen looks promising when Helmholtz coils are used, if construction can be perfected and if a sufficient degree of accuracy and reliability can be reached.