 The important and crucial point characterizing the CCC is the fact that the magnitude of the screening current and its distribution on the surface of the superconducting screen are independent of the position and the path of the wires inside the tube.Typical for a CCC is the use of a SQUID magnetometer as null detector for the magnetic field (SQUID = Superconducting Quantum Interference Device).These are capable of detecting extremely small changes of the magnetic field corresponding to fractions of the magnetic flux quantum = "h"/2"e" ≈ 2×10−15 V·s ("h" is Planck's constant and "e" the elementary charge).