How to make a magnetic field viewer

[Mike Sulzer]

The iron dust in oil is a way to view the fields, and not to measure them.

It can measure them in a crude relative way since the field strength is proportional to the distance between the field lines, although this requires careful interpretation. But i think the point is that viewing the field in this way does not replace measurement and encourages mistakes in understanding.

[Lyrebird Steve]

You can make a pretty good ferro-fluid using Cooking Oil + Laser Printer Toner Powder (generally 40-50% iron).

Then place ferro-fluid into a glass jar full of distilled water. Hours and hours of fun

[Mark Hammer]

Well, suppose you want to experiment with, say, bevelled slugs or polepieces, or perhaps slugs of different composition, or polepieces of different stagger, or mixed-magnet polepieces. The shape of the field created may be useful info, and the relative strength of the field might also be useful info. As others note, this is NOT the same as having a gauss-meter, but one ought to be able to see that THIS has a stronger field than THAT, if the difference is supra-threshold enough.