1. The problem statement, all variables and given/known data A long air-cored solenoid has two windings wound on top of each other. Each has N turns per metre and resistance R. Deduce expressions for the flux density at the centre of the selenoid when the windings are connected (a) in series, and (b) in parallel, to a battery of EMF E and of negligible internal resistance. (In each case the magnetic fields produced by the currents in the two windings reinforce.) 2. The attempt at a solution Maybe the formula for the flux density should be B = (μ0 N I) / (2 r), where N is the number of turns on the coil, I is current and r is the radius of the coil? Though I don't quite understand what is being given. Something like this that has two metal cables wound around it? How can they be connected in series or in parallel?