What is an Oersted and Why Should You Care?

By Dr. Brad Paulson, Thor Engineering, ICMA Standards Representative

When considering the coercivity of magnetic stripes, it may be helpful to think of lights. Suppose you had two lights, one a hand-held, battery operated type and one used by an automobile to illuminate the road ahead. Now, suppose the hand-held light was one of the new LED designs, so that it was as bright as the automobile headlight. With the hand-held light, it would be relatively effortless to point the light to the north, then rapidly flip it to point south, and rapidly flip it to point north again. However, it would be significantly more difficult to flip the automobile rapidly to point the light in the opposite direction and then flip it back again. Clearly, the intensity of the light is independent of the force required to move the light.

So, you ask, what does that have to do with magnetic stripes? The same independent relationship exists between the read voltage and the coercivity of a magnetic stripe. The read voltage of a magnetic stripe corresponds to brightness; the magnetic field strength required to flip the direction of that read voltage corresponds to the coercivity of the stripe. Just as the strength required to flip the light is influenced by whether it is a hand-held light or an automobile headlamp, the coercivity of magnetic stripes can vary widely without variations in the read voltages.

The point of this illustration is that in encoding data on a magnetic stripe, the magnetic polarity of the minute magnetic particles must be quickly flipped back and forth in opposite directions, flipping the read signal, as the head moves along the stripe. These changes in read voltage are used to encode zero and one bits on the stripe, and the higher the coercivity of the magnetic particles, the stronger the magnetic field that is required to flip them. The particles will stay flipped, or encoded, until another magnetic field of the same or greater strength, but opposite in direction, comes along and realigns them, and this is the practical reason to be familiar with coercivity: Coercivity determines how strong a magnetic field is necessary to affect data encoded on the stripe, and, as a consequence, the susceptibility of the data to damage by exposure to common magnets.

Coercivity is measured in a unit called Oersted, named after the 19th century Danish physicist Hans Christian Øersted. The low coercivity ISO standard, ISO/IEC 7811-2, was developed using a 300 +/- 30oersted reference material. In contrast, the high coercivity ISO standard, ISO/IEC 7811-6, was developed using a 2750 +/- 100oersted reference material; thus, the high coercivity magnetic stripe would require a magnetic field about 9 times greater to encode, or erase, the stripe, than would the low coercivity magnetic stripe, making high coercivity magnetic stripes more resistant to accidental erasure than low coercivity material.

To illustrate the resistance of magnetic stripes to accidental erasure, the average read voltage of four encoded cards were measured after exposing them to magnetic fields of different strengths; as seen in the legend of the figure, the magnetic stripe of one card was 300oe, the magnetic stripe of the second card was 2750oe, the magnetic stripe of the third card was 4000oe, and the final card had a debitek® stripe. Notice that the 300, 2750, and 4000oe magnetic stripes initially have an encoded amplitude of about 1, or 100% UR; this is indicative of magnetic stripes that are compliant to Parts 2, 6, and 7 of ISO/IEC 7811. In comparison, the debitek® stripe has an initial amplitude of about 1.6, or 160 percent UR, which is not an ISO compliant magnetic stripe.

known as the gauss, named after the German mathematician and physicist Carl Friedrich Gauss. As seen in the figure, the read amplitude of the 300oe encoded magnetic stripe begins to degrade when exposed to magnetic fields in excess of about 200G. On the other hand, the encoding on the 2750oe magnetic stripe begins to degrade at about 2000G, and the encoding on the 4000oe and debitek® stripes begin to degrade at about 2500G. Clearly, the high coercivity magnetic stripes are more resistant to accidental erasure than the low coercivity magnetic stripes. Since magnetic field strength of common household magnets can be 100G, it is easy to see how introduction of high coercivity magnetic stripes drastically reduced the occurrence of accidental erasures of encoded cards.

Brad Paulson, Ph.D., is the ICMA Official Standards Representative and concurrently serves as principal and founder of Thor Engineering, a consulting company he started in 2001 to test and evaluate media and materials to determine failure modes and recommend material and process improvements. Views expressed here are his own. Questions? Contact Brad at Tpaulson@rconnect.com.