Operating principle of cryogenic current comparator.  Any imbalance in the currents carried by the two conductors I<SUB>1</SUB> - I<SUB>2</SUB> causes an equal and opposite screening current I<SUB>S</SUB> along the surface of the CCC shield.  In reality, this screening current is distributed around the tube and the diagram only shows a representative path. On the outside, far away from the ends, the surface current is distributed symmetrically and can be detected by a detection coil (not shown).