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.
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
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 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.