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I'm aware of a number of schemes to concentrate a magnetic field inside an area. Most obviously, the total field strength inside a solenoid is much greater than the total field strength outside of it. Is the reverse possible?

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Consider a hollow cylinder with current running along it. The inside has no magnetic field but the outside has magnetic field.

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  • $\begingroup$ Huh. It looks like (for an infinitely long cylinder anyway) half the energy goes into shell and half to the space outside. $\endgroup$ – imuli Apr 19 '17 at 12:23

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