The Physics
Factbook
An encyclopedia of scientific essays

Magnetic Field of a MetroCard

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We conducted this experiment in order to find the magnetic field on the surface of a MetroCard. The magnetic strip is created by encasing tiny spikes of iron in a plastic coating. The iron is then aligned in the direction the MetroCard will be scanned. Since the strip is small and the iron is sparse, the magnetic field is tiny.

We knew that the MetroCard was separated into three magnetic tracks, and we wanted to find the magnetic field of each track individually, as well as find the magnetic field of the MetroCard as a whole. We looked in an article written by Redbird in the Spring 2005 edition of 2600 magazine, entitled "Magnetic Stripe Reader", and we were able to locate the tracks. The first track is 0.223" from the bottom of the card, the second is 0.333" from the bottom, and the third is 0.443" from the bottom. We cut the MetroCard accordingly.

We then measured the magnetic field of each of the tracks, and the whole card. The magnetic field was found using a LabPro lab interface device, the program LoggerPro, and a Hall potential difference magnetometer, which measures magnetic field. We slowly moved the device across the magnetic strips, obtaining a graph of magnetic field vs. time. The following are the graphs of the total MetroCard, the first track, second track, and third track, accordingly.

In conclusion, the magnetic field of each of the tracks, as well as the MetroCard itself, was barely above 0, but it did exist. Certainly not strong enough to be called a powerful magnet in any stretch of the imagination, however it's enough of a field to allow the MetroCard to be read, and enough of a field for this experiment.

Gafei Szeto, Daniel Saronson, Michael Robbins -- 2005