I've studied this a little more. All of the example that I came across involved a changing magnetic field - which I think would imply an alternating or pulsating current. However, coaxial cable might be an exception - I'm not sure, because the magnetic field produced appears to have a nonzero curl. I haven't figured out all of the possible shapes that a wire can't have in order to not be considered an inducer, but I'm thinking that the geometry of the wire - which I think in some cases might resemble a helix, likely has something to do with it. I suppose I could try to figure this out a little more by calculating the size of the magnetic field, and where the magnetic field intercepts the other parts of the inducer. The thing is that the magnetic field is going to be intercepting a helical inductor at so many different angles, I imagine that the calculations could become complicated. One question that I thought of is whether or not a changing magnetic field causes induction in the location where the magnetic field is produced in addition to other locations. Also, it seems curious to me - if I remember correctly, that the formula for inductance didn't factor in the shape of the inductor. Maybe this was because it was for a specific type of inductor. I think that the main idea about an inductor is that it produces a magnetic field that alters the current. In AC, I remember reading in Giancoli's Physics for Scientists and Engineers, when AC is applied to an inducer the effect is create a current when the current is slowing down and decrease the current when it is speeding up, making the current alternate less.