Magnetic fields are generated by magnetic materials but they cannot be seen. Physics expresses them as closed loop lines with direction. In real life we can make them visual with ferric fluids or iron filings. But when speaking of electronics it is more interesting to see magnetic fields digitally – or simply measure it.
To do so a magnetic field sensor is needed. Today it isn’t a problem because lots of miniature sensors are available for everyone to purchase. In this particular project evilmadscientist used the Honeywell HMC5843 3-Axis magnetoresistive sensor that can read magnetic fields up to 6 Gauss. So it is limited to sense pretty weak magnetic fields and can be used near magnetic poles of magnets. Magnetic field sensor has been interfaced to Arduino along with Processing that reads data and plots them on screen. As sensor measures three axis – it produces a 3D pixel on XYZ plot. By spinning it around you can get interesting plots.