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The Photonics Spotlight

Resolution of Conundrum: No Magnetic Field on the Axis of a Coil

Dr. Rüdiger Paschotta

Some people suspected that this strange issue is related to some subtleties associated with the concept of infinity – field lines going to infinity might leave some room for surprises. However, the resolution is really not that subtle: it is the uncovering of a simple mathematical trick. It is true that the magnetic field strength has no x and y components on the symmetry axis, but from this it doesn't follow that ∂Bx/∂x= ∂By/∂y= 0. Therefore, ∂Bz/∂z doesn't have to be zero, and the whole argument breaks down. At least in this case, we don't have to rewrite the textbooks!

It is surprising how easily one falls into this trap. It happened to professors (not stupid ones), so never mind if it happened to you!

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

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