The evaluation of the magnetic field inside and outside a uniform current density infinite solenoid
of uniform cross-section is an elementary problem in classical electrodynamics that all
undergraduate Physics students study. Symmetry properties of the cylinder and the judicious use of
Ampere’s circuital law leads to correct results; however it does not explain why the field is non
zero for a finite length solenoid, and why it vanishes as the solenoid becomes infinitely long. An
argument is provided in Farley and Price (2001 Am. J. Phys. 69 [http://dx.doi.org/10.1119/1.1362694]
751 ), explaining how the magnetic field behaves outside the solenoid and not too far from it, as a
function of the length of the solenoid. A calculation is also outlined for obtaining the field just
outside the circular cross section solenoid, in the classic text Classical Electrodynamics by
Jackson, 3rd edn (John Wiley and Sons, Inc.), problems 5.3–5.5. The purpose of th…