Apparatus to hold flat neodymium magnets (2 x 1 x 1/16 in.). This creates a moderately uniform magnetic field between the magnets and the whole apparatus can be placed on a scale. Placing a wire segment with a horizontal current flow between the magnets will exert a force proportional to the current according to the Lorentz Force Law (for straight wires):

Force = (current)*(length of horizontal wire) cross-product (Magnetic Field). The wire should be shaped like a U with a flat bottom. In order to stay within the uniform portion of the field the horizontal segment of wire should be ~1 in and turn at right angles upward at the ends so as not to contribute to the force calculation (the two opposite vertical components result in equal and opposite forces in the horizontal direction). This can be used to measure the magnetic field between the magnets. Also the top of the apparatus can rotate so that the dependence on the angle in the cross-product can be seen.

Example magnets this will hold: http://www.magnet4sale.com/n45-2x1x1-16-neodymium-rare-earth-block-magnet/

Notes:

1. The fasteners may break when dropped or if not careful during assembly but they're so small just print an extra.
2. For moderate currents of ~5 amps your scale needs to be sensitive on the order of tenths of a gram.
3. Print orientation is the largest flat surface down, so top piece is right side up, bottom and fastener are upside down, layer thickness 0.15