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Magnetic Field Visualization
Thinking about springs to apply downforce on plotting pen holders suggested magnets, so I extricated some neodymium bars from my collection of power toothbrush heads:

Magnets – single A snippet of magnetic field visualization film shows a dipole pattern:

Magnets – single – field visualization Snapping two of them together in line:

Magnets – in line … produces a quadrupole:

Magnets – in line – field visualization Now, if only I had some magnetic monopoles, this whole thing would be easier!