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Eddy Current Testing

Basic Principles
History of ET
Present State of ET

The Physics
Properties of Electricity
Current Flow & Ohm's Law
Induction & Inductance
Self Inductance
Mutual Inductance
Circuits & Phase
Depth & Current Density
Phase Lag

Eddy Current Instruments
Resonant Circuits
Impedance Plane
Display - Analog Meter

Probes (Coils)
Probes - Mode of Operation
Probes - Configuration
Probes - Shielding
Coil Design
Impedance Matching

Procedures Issues
Reference Standards
Signal Filtering

Surface Breaking Cracks
SBC using Sliding Probes
Tube Inspection
Heat Treat Verification
Thickness of Thin Mat'ls
Thickness of Coatings

Advanced Techniques
Multi-Frequency Tech.
Swept Frequency Tech.
Pulsed ET Tech.
Background Pulsed ET

Remote Field Tech.


Formulae& Tables
EC Standards & Methods
EC Material Properties

Properties of Electricity

Since eddy current inspection makes use of electromagnetic induction, it is important to know about the scientific principles of electricity and magnetism. For a review of these principles, the Science of NDT materials on this Internet site may be helpful. A review of the key parameters will be provided here.


It is well known that one of the subatomic particles of an atom is the electron. Atoms can and usually do have a number of electrons circling its nucleus. The electrons carry a negative electrostatic charge and under certain conditions can move from atom to atom. The direction of movement between atoms is random unless a force causes the electrons to move in one direction. This directional movement of electrons due to some imbalance of force is what is known as electricity.


The flow of electrons is measured in units called amperes or amps for short. An amp is the amount of electrical current that exists when a number of electrons, having one coulomb of charge, move past a given point in one second. A coulomb is the charge carried by 6.25 x 1018 electrons or 6,250,000,000,000,000,000 electrons.

Electromotive Force

The force that causes the electrons to move in an electrical circuit is called the electromotive force, or EMF. Sometimes it is convenient to think of EMF as electrical pressure. In other words, it is the force that makes electrons move in a certain direction within a conductor. There are many sources of EMF, the most common being batteries and electrical generators.

The Volt

The unit of measure for EMF is the volt. One volt is defined as the electrostatic difference between two points when one joule of energy is used to move one coulomb of charge from one point to the other. A joule is the amount of energy that is being consumed when one watt of power works for one second. This is also known as a watt-second. For our purposes, just accept the fact that one joule of energy is a very, very small amount of energy. For example, a typical 60-watt light bulb consumes about 60 joules of energy each second it is on.


Resistance is the opposition of a body or substance to the flow of electrical current through it, resulting in a change of electrical energy into heat, light, or other forms of energy. The amount of resistance depends on the type of material. Materials with low resistance are good conductors of electricity.  Materials with high resistance are good insulators.