Arie,

If you are considering a coil/magnet as for a seismometer, as 
compared to an air-inductor as in a radio transmitter, perhaps
the handiest formula, which I have posted once-upon-a-time, is

G = B * L * 10^6 Newtons/Ampere  (or volts/meter/second)

Where B is the magnetic field in gauss, and L is the total length of
the winding in centimeters. If one plays with the math (from below),
all the other physical dimensions cancel out. You can get L by knowing
the coil resistance and the wire size.

If you know the coil parameters and can measure the generator constant,
this can be used to determine the stength of your magnet(s).

The more elaborate formulas for the generator constant of a multi-layer
coil in a magnetic field are in the appendix of Riedesel: 

"Limits of Sensitivity of Inertial Seismometers with
Velocity Transducers and Electronic Amplifiers";
by Mark A Riedesel, R.D.Moore, and J.A.Orcutt; Bulletin of
the Seismological Society of America, Vol. 80, No. 6, December 1990.

Or would you like me to post the formulas.?

Regards,
Sean-Thomas
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