NDT Method Overview: What is Electromagnetism? 

 

While many of our readers are well-versed in NDT, some might be just learning about our industry. This series will serve as a primer to the major NDT methods covered in ASNT’s publications, educational programming, and events. The following is an excerpt from the second edition of Fundamentals of Eddy Current Testing, published by ASNT in 2022, which has been adapted for this blog post. Eddy current testing is an important technique in the electromagnetic testing method. 

This book was developed to be used by technicians for classroom training, self-instruction, and as a reference. It is available from the ASNT Store both in print and now as an ebook. 

Introduction

The link between magnetism and electricity was discovered in 1824 by Hans Christian Oersted, who found that current in a wire could move a magnetic compass needle outside the wire. A few years later the opposite effect was discovered: a magnetic field in motion can force electrons to move, producing current. This important discovery was made in 1831 independently by Michael Faraday in England and Joseph Henry working in the US. Electromagnetism, therefore, consists of the magnetic effects of electrical current. Electrons in motion have an associated magnetic field; a moving magnetic field can produce current. These electromagnetic effects have many applications that are the basis of eddy current testing. 

A Magnetic Field About a Conductor 

The fact that there is a magnetic field associated with current flowing in a wire is illustrated in Figure 1, where iron filings are attracted to the wire when the current is flowing and are not attracted when the current is stopped. 

The magnetic field is strongest at the surface of the conductor, decreasing inversely as the square of the distance from the conductor increases. 

With circular lines of magnetic force, a north pole in the field would tend to move in a circular path. The direction must be considered in terms of clockwise or counterclockwise rotation. In Figure 2, the compass needle shows a north pole would move counterclockwise in the field. To determine the circular direction of the field, the following rule can be used: looking down the wire in the direction of electron flow, the magnetic field is counterclockwise, as shown in Figure 2. 

Magnetic Polarity of a Coil 

Bending a straight conductor in the form of a loop, as shown in Figure 3, has two effects. First, the magnetic field lines are more dense inside the loop. The total number of lines is the same as for the straight conductor, but in the loop the lines are concentrated in a smaller space. Second, inside the loop, all the lines are adding because they are traveling in the same direction. This makes the loop field effectively the same as a bar magnet, with opposite poles at opposite faces of the loop. 

A coil of conducting wire with more than one turn is generally called a solenoid. An ideal solenoid, however, has a length much greater than its diameter. Like a single loop, the solenoid concentrates the magnetic field inside the coil and provides opposite magnetic poles at the ends. These effects are multiplied, however, by the number of turns. Referring to Figure 4a, note that the magnetic field lines aid each other in the same direction inside the coil. The field is strongest at the center. Outside the coil, the field corresponds to that of a bar magnet, with north and south poles at opposite ends of the solenoid, as illustrated in Figure 4b. 

Magnetic Induction 

Because electrons in motion have an associated magnetic field, when a magnetic flux moves, the motion of magnetic lines cutting across a conductor forces free electrons in the conductor to move, producing current. The process is called induction because there is no physical connection between the magnet and the conductor. The induced current is a result of generator action as the mechanical work put into moving the magnetic field is converted into electrical energy when current flows in the conductor. Without motion, there is no current. 

The motion is necessary to have the lines of the magnetic field cut across the conductor. This cutting can be accomplished by motion of either the field or the conductor. 

To have electromagnetic induction, the conductor and the magnetic lines of flux must be perpendicular (normal) to each other so that motion makes the flux cut through the cross-sectional area of the conductor. As shown in Figure 5, the conductor is at right angles to the lines of force in the field H. The reason these must be perpendicular is to make the induced current have an associated magnetic field in the same plane as the external flux. When the magnet is moved downward, current flow is in the direction shown (A toward B). If the magnet is moved upward, current will flow in the opposite direction. 

Consider the case of magnetic flux cutting a conductor that is not a closed circuit, as shown in Figure 6. The motion of the flux across the conductor forces free electrons to move, but, with an open circuit, the displaced electrons produce electric charges at the two open ends. For the direction shown, free electrons in the conductor are forced to move to point A, and electrons accumulate there. Point A then develops a negative potential. At the same time, point B loses electrons and becomes positively charged.

The result is a potential difference across the two ends, provided by the separation of electric charges in the conductor. The potential difference is an electromotive force, generated by the work of moving the flux. The amount of induced voltage produced by flux cutting the turns of a coil depends upon the following three factors: 

• Amount of flux. The more magnetic lines of force cutting a conductor, the higher the induced voltage is. 
• Time rate of cutting. The faster the flux cuts a conductor, the higher the induced voltage. 
• Number of turns. The more turns of a coil, the higher its induced voltage. 

The induced voltage can be calculated in volts from the formula: 

where 

θ is maxwells or number of lines, 
T is time in seconds, and 
N is number of turns. 

_______

Fundamentals of Eddy Current Testing, second edition, provides a thorough examination of the basic theory and principles of eddy current testing and builds on the original book written by Donald J. Hagemaier. It is packed with new color photographs and illustrations as well as updated standards and practices, and includes two new chapters on pulsed eddy current and eddy current array. The main topics covered include electrical theory; electromagnetism; inductive reactance and impedance; eddy current test principles; coils, instruments, and standards; and impedance-plane response. 

For additional information on electromagnetic testing and eddy currents, check out these ASNT resources:

Books available from the ASNT Store

• Nondestructive Testing Handbook, Vol. 5: Electromagnetic Testing, third edition
• ASNT Level III Study Guide: Electromagnetic Testing, third edition
• Personnel Training Publications: Electromagnetic Testing (ET), Classroom Training Book, second edition

 

ASNT Webinars and Refreshers

• Using Eddy Current Arrays to Augment MT and PT – Webinar Recording
• Eddy Current Inspection for Stainless Steel Welds – Webinar Recording
• Refresher/Certification Prep Courses