You've probably seen films where individuals move magnets close to iron dust, causing the particles to line up with the magnetic field patterns. In order for magnetic viewing film to function, a gap between two sheets of clear plastic must contain an oil suspension of dust. One may virtually see the magnetic fields on an LCD screen by sliding that film across magnets.
Sivers accomplished this by placing a thin sheet of magnetic viewing film over four wheels in a 3D-printed frame. They put numerals in a pattern like an analogue clock face all the way around the four wheels. As the wheels turn, the magnetic fields of the numbers, which are constructed of magnetic foil, are visible on the viewing film. Reading this magnet clock is as simple as reading a digital clock since it has four boxes on the viewing film that are stencilled with the digits that make up the current time.
Source - Instructable