Magic cylinder adjustable in field strength

A permanent magnet structure comprising:

• a first shell of magnetic material forming a cylindrical hollow cavity, said magnetic material of said shell being magnetized such that said first shell produces a first working magnetic field in said cavity, said first working magnetic field having a given magnitude and direction in said cavity;
• a second shell of magnetic material positioned concentrically within said first shell, said magnetic material of said second shell being magnetized such that said second shell produces a second working magnetic field in said cavity, said second working magnetic field having a given magnitude and direction in said cavity;
• said first and second working magnetic fields combining in said cavity to produce a first composite working field having a given magnitude and direction, said first and second shells being rotatable with respect to each other such that said given magnitude and direction of the first composite working field can be adjusted by rotating said first and second shells with respect to each other;
• a third shell of magnetic material positioned concentrically within said second shell, said magnetic material of said third shell being magnetized such that said third shell produces a third working field in said cavity, said third working field having a given direction in said cavity;
• a fourth shell of magnetic material positioned concentrically within said third shell, said magnetic material of said fourth shell being magnetized such that said fourth shell produces a fourth working field in said cavity, said fourth working field having a given direction in said cavity;
• said first, second, third and fourth shells are magic rings;
• and
• said third and fourth working fields combining in said cavity to produce a second composite working field having a given direction in said cavity, said third and fourth shells being rotatable with respect to each other such that the given direction of the second composite working field can be adjusted by rotating said third and fourth shells with respect to each other, said first and second composite working fields combining in said cavity to form a final composite working field having an adjustable magnitude.