Designing an Electromagnet for a Digital Clock
rotate your electromagnet by 90°, so that its axis is perpendicular to the plexiglass. That's kind of obvious – it puts only one of the filings-attrackting ends close to the glass.

If you wind your magnet around a rectangular-crosssection piece of iron, then you can make "straight lines". Notice that even these lines will be feathered – the filings will align along the magnetic field lines, and those form a convex shape from one end to the other