The Cryo Current Comparator as a Calculable DC Ratio Standard

Experience with Ac Calibration of Conventional CCC

“…Main property of CCC is that dc ratio error does not depend on position of the wires inside superconducting sheath. By designing the geometry of CCC to get high attenuation of the leakage magnetic field, the dc ratio errors below 10-'o can be achieved [5,6]. The verification of the calculation is confirmed by the measurements of the deviation from 1:1 ratio and followed by binary step-up procedure.…”

“…1. They are: a) overlapped-tube type, Type I [4], and b) "toroidal" type, Type II [5]. When dc currents I1 and '2 are fed into ratio windings N, and N2, in opposite direction, the balance of ampere-turns is fulfilled at condition IIN, -12N2 + dd,= 0, where d& is deviation from 1:1 ratio (if NI = N2).…”

“…Analysis of the influence of positions of the ratio windings in Type II CCC [5] showed that its dc error depends on the position of pick up coil and the dimensions of the gap. AC ratio error.…”

SQUIDs for Standards and Metrology

“…This leakage flux carries the information on the position of the windings. Theoretical error values of the order of one part in 10 10 or less have been reported for both types of CCC [39]. …”

“…The type II geometry is the most suited for ac applications of CCC. Errors reduced to one part in 10 8 at 10 kHz have been reported [39]. For a type I CCC, a simple approximate expression for the error on the current ratio I 1 /I 2 can be given, in terms of the self-inductance of the windings L 1 and L 2 and their mutual inductance M = k (L 1 L 2 ) 1/2 with a coupling factor k…”