Impedance is the opposition to current flow and is a two dimensional parameter consisting of resistance and reactance.
Resistance is the opposition to the flow of both direct and alternating current. Reactance is the opposition to flow of
alternating current only. Reactance can be either capacitive or inductive. Both resistance and reactance are measured
in ohms. Of primary interest in eddy current inspection are resistance and inductive reactance, the latter due to
inductance of a coil. Capacitive reactance becomes significant in only a few cases and will be discussed later. The
impedance of a test coil is related to the current flow in and voltage drop across the coil as follows:
Z = E / I
Impedance of coil (ohms)
Voltage drop across the coil (volts)
Current through coil (amperes)
When AC or DC flows through a purely resistive element (i.e., a straight section of wire or a carbon resistor) of an
electrical circuit, the impedance is resistance only and is expressed as:
R = E / I
Voltage drop across the resistor (volts)
Current flowing through circuit (amperes)
In an AC circuit containing resistance only, the voltage and the current are in phase. The term "in phase", when used
to describe the relationship between the voltage and current, indicates that changes in current occur at the same time
and in the same manner (direction) as changes in voltage. Figure 4-13 is an example of two quantities that are in phase.
Figure 4-13. Sinusoidal In-Phase Variation of Alternating Current and Induced Magnetic Field.
The inductance of an eddy current probe is the result of magnetic field effects of the alternating electric current in the
probe. Inductance is a measure of the capability of a circuit to induce current flow in another circuit. It is proportional
to the ratio of the magnetic flux (f) linking (encircling) a circuit to the current (I) that produced the flux. When the
flux from one inductor is linked to (passes through) another inductor, the inductance is called mutual inductance (M).
An electrical transformer is an example of a device where M is a significant parameter. For eddy current testing, we