the d.c. resistance of the shield. If the frequency of these currents increases, the
coupling impedance will continuously decrease if the shield is of rigid crosssection;
then the eddy currents will attenuate the current density at the inner
surface of the cylindrical shield. Hence rigid or corrugated shields, i.e. flexwell
cables, are best suited for noise reduction. For braided shields, the coupling
impedance is in general not a stable quantity, as the current distribution within
the shield is likely to be influenced by resistive contacts within the braid.
Double-shielded cables with predominantly two insulated braided shields will
improve the behaviour. Best conditions are gained by placing the coaxial
cable into an additional, non-braided metal tube, which is connected to ground
potential at least at the input end of the measuring cable and also at its end.