Maxwell coil
A Maxwell coil is a device for producing a large volume of almost constant (or constant-gradient) magnetic field.
A Maxwell coil is an improvement of a Helmholtz coil: in operation it provides an even more uniform magnetic field (than a Helmholtz coil), but at the expense of more material and complexity.
Description
A constant-field Maxwell coil set consists of three coils oriented on the surface of a virtual sphere[1]. According to Maxwell's original 1873 design [2] each of the outer coils should be of radius 
and distance 
from the plane of the central coil of radius 
. The number of ampere-turns of each of the smaller coils should equal exactly 
of the middle coil.This arrangement removes variations in magnetic field, up to its 6th-order derivative with respect to position, near the centre of the virtual sphere.
A gradient-field Maxwell coil is essentially the same geometry of the 3-coil configuration above, with the central coil removed to leave only the smaller two coils [3]. If the current in one of the coils is reversed, a uniform-gradient magnetic field is produced near the centre of the two coils.
References
- ^ Garrett, Milan Wayne, Journal of Applied Physics, 38(6), pp2563-2586
- ^ Clerk-Maxwell, James (1873). Treatise on Electricity and Magnetism. Oxford: The Clarendon Press. ISBN 0-486-60636-8. Page 319.
- ^ R. Pascone, Manhattan College, T. Vullo and P.T. Cahill, (1993) http://ieeexplore.ieee.org/iel4/1093/8547/00701859.pdf