Today's measurement is based on the lifting force from current in a voice coil.

**Flux measurement by integration of induced voltage will be done next.**

The lifting force factor, in newtons per ampere, is identical to a loudspeaker parameter called Bl, in tesla-meters. Product of flux density B and the length of wire in the magnet gap. Ain't the SI system nice here? Some presentations found on the Internet suggest that 20 T-m is an ordinary value in powerful woofer drivers. Anybody here got experience with loudspeaker design or reverse-engineering?

My hobby lab style, as some friends know, often seeks quick gratification from materials and tools on hand. Buying new stuff, or having to clean up and find lost stuff, is no fun.

Today's "voice coil" started with a tube wound from 1-inch-wide paper, and some glue, using copper pipe as a mandrel. It has 12 turns of 26 AWG magnet wire, from a 5-lb spool that I _did_ find under cobwebs and dust.

It's driven by an adjustable DC power supply, whose digital current meter has a resolution of 0.01 ampere.

The resisting force is a stack of nickels (USA $0.05 coins) which are taken to be 5 gram weights. Picture also shows that the ferrite ring magnets are far from concentric with the steel pole parts. I drew an eccentric circle on top plate directly above the inner edge of the ferrite parts. Is that just from sloppy assembly, or is it on purpose? Could be to leave more space for some other part in original Sunfire powered subwoofer box. Like the eccentric jet engine inlets on 737's, which are to increase the ground clearance.

For N_nickels = 0 to 4, I recorded the current at which voice coil began to rise from its support.

To get the most out of the few significant digits, I let Excel figure a straight-line fit: The slope works out to 1.26 N/A, which we take as the Bl value.

The average coil diameter, times pi, times the number of turns, gives us wire length l = 2.107 meters. So we infer that B = about 0.60 teslas, around the middle of the gap, at the radius of this coil.

The flux density in this magnet will be about 27% greater at the inner pole face than at the outer pole face, simply because of the radial geometry. But the force factor, in N/A, doesn't depend on the radial position of any turn in a voice coil. A turn near the outside is in a weaker B field, but has a proportionately greater length of wire, so contributes the same amount toward total "thrust". It crosses the same total amount of magnetic flux (in webers or maxwells) per millimeter of axial displacement.

This same post just went up on another forum for hobbyists, but no more than that.

[edit]Got the sign without finding a compass, or applying right or left hand rule. The magnet assembly _is_ the compass. Suspended with axis horizontal, it turns so the annular plate (outside pole) faces magnetic south.

The inner pole is connected to end plate which faces magnetic north.

Will be fun to model this in FEMM, with enough room on the outside to see and measure the far-field dipole moment.