The term electrical conductor is used for all materials which have charge carriers (such as valence electrons in metals), which can conduct the electrical current. Electrical conductivity? (sigma) – aka conductance value – is a material-specific parameter. It describes how well a material can conduct an electrical current. The inverse of specific conductivity is specific resistivity? (rho). It expresses the amount of resistance a material exerts against the flow of charge carriers. To determine these material parameters the probe geometry (length and cross-sectional area) and electrical quantities (voltage drop and current strength or resistance) are linked together:
σ – Specific conductivity in S / m
(Siemens / m, 1 m / Ω mm2 = 1 MS / m) ρ – Specific resistivity in Ω mm2 / m
U – Voltage drop in V (volt)
I – Current strength in A (ampere)
R – Ohmic resistance in Ω (ohm)
– Length of the conductor in m
A – Cross-sectional area of the conductor in mm2
In Anglo-American regions electrical conductivity is indicated using the IACS system.
The specific conductivity, and thus also the specific resistivity, are temperature-dependent.
In the case of metals specific conductivity generally drops with rising temperature, because the increasing thermal movement of the atoms generates more resistance to the flow of the charge carriers.
Impedance of coils
In eddy current testing one uses a coil to generate (induce) eddy currents in the product to be tested and record the feedback from the product, which reflects the product’s properties.
In the simplest case, an eddy current probe consists of only one coil, which acts as both sender as well as receiver (parametric probes). The operational principle consists of the product to be tested impressing its properties on the coil – or to be more precise, on the coil impedance.
Impedance is understood as resistance to alternating current. In a coil it is composed of 2 components: The ohmic resistance R (this corresponds to the DC resistance of the coil wire and is constant; i.e. independent of frequency) and the inductive reactance XL, which is produced because the coil wire is wound into turns. Provided a coil is traversed by an alternating current, the coil turns are within the area of influence of their own magnetic field. Consequently, currents are induced in them which flow in opposition to the causative coil current and superimpose themselves on it. The resulting overall current is thus phase-shifted; i.e. time lagged.
Ohmic resistance is dependent on the geometry and material of the conductor wire:
– R – Ohmic resistance in Ω
– l – Length of the conductor in m
– A – Cross-sectional area of the conductor in mm2
ρ – Specific electrical resistivity in Ω mm2 / m
The inductive resistance XL is greater:
– The higher the frequency f, and
– The larger the inductivity L of the coil
and can be calculated with the following formula:
XL – Inductive reactance in Ω
f – Frequency in hertz (Hz = 1 / s)
L – Inductivity in henry (H).
The coil inductivity is dependent on the coil turns, the coil dimensions and the material forming the interior of the coil:
L – Inductivity in H (henry, 1 H = V s / A)
μ – Magnetic permeability in the coil interior in V s / A m (with μ = μ0 * μrel)
n – Coil turns
A – Cross-sectional area in mm2
l – Coil length in mm
