found this

Title: The Falling Coil Applet
[img]http://web.mit.edu/8.02t/www/802TEAL3D/pirelli_images/fallingcoil200.jpg[/img]
An interactive Java 3D applet field shows the field configuration around a non-magnetic ring as it falls under gravity in the field of a fixed magnet. In the initial configuration the coil has no resistance and levitates in the field of the magnet. Hit run and let the ring bounce several times. At the bottom of a bounce hit the "iron filings" box to see a representation of the complete field. Hit run again and increase the resistance to "10" using the slider. Watch the subsequent behavior of the ring.

The Ejs applet above is fantastic! now with field lines too! thanks Prof Hwang. i will download and learn  ;)

http://web.mit.edu/8.02t/www/802TEAL3D/teal_tour.htm
this The Falling Coil Applet by MIT [img]http://web.mit.edu/8.02t/www/802TEAL3D/graphics/navbars/topnav/physics802.gif[/img] has a special button on the middle right, it is called magnetic field lines by [color=blue][b] iron filing[/b][/color].
i think it is drawn using a techniques called line "Dynamic Line Integral Convolution for Visualizing Streamline Evolution"
The paper is here
http://icampus.mit.edu/projects/Publications/TEAL/DynamicLineIC.pdf

The way they did it was by using a plane, to allow such a visualization [img]http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=1449.0;attach=3356;image[/img]. I can't understand the mathematics, maybe you can explore if it can be done on Ejs! Thanks!

Pedagogy wise, it is a good visualization feature to have, because it allow students to relate back to real life "iron filing" representation of magnetic field lines.