Field attenuation as the underlying principle of cryo-current comparators

The connection between the error of a cryo-current comparator and the magnetic field attenuation in the gap region of its superconducting shield was treated for the special but very common case of co-axial gap regions in a previous paper (Grohmann et al. Cryogenics; 16: 423 (1976)). This paper extends the error analysis on gaps forming a ring cavity. Such gap elements occur in toroidal cryo-current comparators as a link between two adjacent co-axial cavities. The results for these comparators are that the only field component which may penetrate the gap without attenuation is the one containing the useful signal about the current linkage of the comparator. All other field components contribute to the error of the comparator. They are attenuated according to their degree of symmetry. As the error functions are known it is now possible to construct cryo-current comparators with a predictable upper limit to their dc error. (author)