Does anyone have formulae to calculate the number of turns required on a coil to produce 10V at approx 5W. I wish to have a coil spinning on a 16" diameter former in a 0.25 gauss magnetic field at a minimum of 2.2Hz. I need to be able to work out the size of coil and number of turns to produce approx 5W at 10-15V which will be DC rectified (frequency is not important). The former is provided with a mechanical rotational force of between 2.2 - 16.6Hz. The magnetic field will vary between 0.25 - 0.65 gauss, but will be constant and parallel across the 16" former.