So I was reading a thread on PF talking about the radius of a solenoid and Bob S answered with a link: On that site, it states that the Radius = the outer radius - the inner. It doesn't make sense. What happens when there is only one layer of coil and the outer radius (OR) is basically the inner radius (IR). That would create a larger magnetic field than if I had multiple layers of coils and the OR > IR which would then create a smaller one. I don't understand why.