This class of material (essentially represented by Ni, Fe, Co) is marked by its relative Permeability µr, which is intrinsically greater than 1. The permeability determines the connection between magnetisation H and induction B according to the relationship B=µ0•µr•H her. µr is indeed a material constant but generally, especially with ferromagnetic material, unfortunately not a constant. On the basis of the physical causes of the high permeability, µr is associated for transmission with two extremely unpleasant properties, namely saturation flux density and hysteresis.
 
The first property means that after exceeding a certain magnitude of magnetisation or of the causative current, µr assumes the value of 1 fairly abruptly and with that, nonlinearity similar to that shown by a diode. The second one means that the induction depends not only on the instantaneous value of the current but also on its direction (increasing or decreasing) and so ‘induces’ a further non controllable nonlinearity in each of B’s or rather H’s influenced signal paths. The influence occurs over the magnetic alternating field of the signal flow, which induces in the conducting materials in its range voltages or rather currents and leads to frequency dependant (linear distortion) eddy current losses. If these losses are caused in ferromagnetic materials, the above mentioned nonlinear distortions become part of the signal. The only possibility to control these nonlinearities is not to allow them to occur in the first place. For reason of construction, it is not possible to entirely avoid eddy current influences however the use of ferromagnetic material can be avoided. A magnetisable metal is, for example Nickel. Gold plated surfaces are usually applied on top of a nickel layer since it acts as a primed surface between the metal body and the gold plating. In order to prevent the above mentioned distortions, WBT uses a complex nickel free gold plating for its top quality products.