Magnetic Field Viewing Film

Magnetism is surely one of the most mysterious phenomena of the material world; its invisible forces of attraction and repulsion delight and enthral us with childlike fascination no matter what our age. The region of magnetic influence around a magnet is called its magnetic field. There are many ways to ‘see’ magnetic fields, even though they are invisible to the human eye. When iron filings are sprinkled near a magnet, those metallic shards that are closest to the magnet will cling to its poles, where the force is strongest, and those filings that are further away will align with the magnetic flux lines, revealing the forces at play. The more densely-gathered the iron filings, the stronger the magnetic field in that place. Other substances can reveal a magnet’s force too; the shape of magnetic liquids such as ferrofluids depends on the intricate balance between the liquid’s surface tension and the shape of a nearby magnetic field. Whilst we as humans have no way of detecting magnetic fields, many species are sensitive to the magnetic field of the Earth, such as migrating birds, dolphins, whales and honeybees.

This sample of thin, translucent and flexible film provides us with a sophisticated way to visualise magnetic fields. Place it near a magnet, and you’ll be able to see the outline of the magnetic field produced by the magnet. It works because the plastic sheet contains a coating of tiny capsules which contain flat flakes of nickel suspended in oil. Since nickel is a magnetic material, the flakes will interact with magnetic fields that come close to the sheet. When the magnetic field acts parallel to the surface of the sheet, the flakes appear bright green. When the field acts perpendicular to the sheet, as would be the case when the sheet is placed on a magnetic pole, the flakes re-orientate so that they are viewed edge-on, and they appear dark green.

Visualisation in this way to aid our detection and understanding of the wily ways of magnetic fields has enabled us to harness them in vital technologies. The first example of this was the lodestone compass, a rock of magnetite (a naturally occurring iron ore, Fe3O4) known to the ancients which aligned with the Earth’s magnetic field. More recently, both electric motors and generators rely on moving magnetic fields to generate electric current, and this interaction between magnetic field and metallic coil is also central to the workings of microphones and loudspeakers.