I have a solenoid with 180 turns, which is 10 cm long. It has a half core of magnets (which could be constituted as part iron core). The magnetic field strength was measured to be 0.004982 T inside the solenoid around the middle when 12v was run through it and 0.004512 without power running through it. To work out the theoretical magnetic field, I have to use Ampere's law: B=μ x N x I B = 8.75x10^-4(because of the iron core)x180x(current in amps) The problem is I don't have the current in amps. But I thought I could use Ohm's law: V= IR and changed it to I= V/R I need the resistance without measuring it, but I don't know how to get it.