My (high school, gah!) text book gives an experiment: I take a straight current carrying conductor, a cardboard sheet is placed perpendicular to the conductor, so that the conductor passes straight through the sheet, remains perpendicular. Then I use a salt sprinkler to sprinkle iron filings on that board, push in the key for that circuit of the conductor and tap the board gently. The textbook diagram shows that the iron filings line up in concentric circles around the conductor. While it should be correct to show field lines in that manner since we're considering an individual field line, it shouldn't be that way for iron filings. If a cross section of this concentric pattern is analyzed for the field, it will turn out to be a wave-form rather than a curve. Since the field B is supposed to be directly proportional to the current and inversely proportional to the distance from the conductor, the latter should prevail. I cannot make a sane justification for the concentric pattern of circles.