Measuring Magnetic Properties of Materials (Including Your Hand) with an Electronic Balance

featuring contributions from Paul Lee and John Tian

All materials display some magnetic properties depending on such factors as their electron arrangement, temperature, and particle size/orientation. Three notable types of magnetism displayed in matter are diamagnetism, paramagnetism, and ferromagnetism. Diamagnetic materials have orbitals containing only paired electrons; each occupied orbital contains two electrons in opposing spin states. They tend to be nonmagnetic or slightly repelled by magnetic fields. Paramagnetic materials have some orbitals with unpaired electrons, each containing only a single electron with a single spin state. These materials are attracted to magnetic fields, but their random unpaired spin orientations only enable weak attractions.  Ferromagnetic materials have unpaired electrons similar to paramagnetic materials. However, the unpaired electron spins of the ferromagnetic materials are aligned cooperatively in magnetic domains, which enable much stronger attraction to a magnetic field than in simple paramagnetic materials.

It is possible to measure the relative strengths of magnetic properties in materials with simply an electronic balance (that should have draft doors and measure to tenths of milligrams), a strong magnet (preferably a neodymium-iron-boron magnet), and two polystyrene foam cups. This demonstration is based on that described in Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky; G. C.; Robinson, W. R. Teaching General Chemistry: A Materials Science Companion; American Chemical Society: Washington, DC, 1993.

First, level the balance in a stable area, with few outside forces that can disrupt the balance. Next, place both cups on the balance pan with both openings facing each other forming a diamond-like structure. Place the magnet on top of the cups to isolate it magnetically from the balance mechanism.  The top of the magnet should be only a millimeter or two under the top draft door. Cut the cups smaller if the apparatus does not fit within the draft doors of the balance. Finally, with all draft doors closed, tare the balance to cancel out the mass of the cups and magnet. To measure the magnetic behavior of substances, place samples on top of the top draft door.  Paramagnetic and ferromagnetic substances will attract the strong magnet upward away from the balance pan, resulting in a negative reading on the balance display.  Diamagnetic substances will repel the strong magnet toward the balance pan, resulting in a positive reading on the balance display. It was amazing to us when we observed that even a hand held over the draft door over the magnet without touching the door will cause a slight positive reading on the balance display! We presume that water and other diamagnetic substances in human hands act to slightly repel the magnet.