1. The problem statement, all variables and given/known data I'm doing an experiment to determine the charge/mass ratio of the electron, and one of the questions that I need to answer is to deduct the formula for the magnetic field depending on the radio on the center of the pair of coils. Figures, formulas and description of the experiment: http://www.feg.unesp.br/~algatti/laboratoriodeestruturadamateria/RotMedidasCargaMassa-2009.pdf It is in portuguese, sorry... 2. Relevant equations The relevant equations for the problem are here: http://dl.dropbox.com/u/18903527/IMG01374-20120319-0224.jpg [Broken] I need to deduct the first one on the page using the last one, all the information are in the middle. 3. The attempt at a solution I tried to substitute the values of r and θ in B_r and B_θ and play with the math, but it isn't working.