A Helmholtz coil is a device for producing a region of nearly uniform magnetic field, named after the German physicist Hermann von Helmholtz. It consists of two solenoid electromagnets on the same axis. Besides creating magnetic fields, Helmholtz coils are also used in scientific apparatus to cancel external magnetic fields, such as the Earth's magnetic field.
A Helmholtz pair consists of two identical circular magnetic coils (solenoids) that are placed symmetrically along a common axis, one on each side of the experimental area, and separated by a distance equal to the radius of the coil. Each coil carries an equal electric current in the same direction. 
Setting , which is what defines a Helmholtz pair, minimizes the nonuniformity of the field at the center of the coils, in the sense of setting  (meaning that the first nonzero derivative is as explained below), but leaves about 7% variation in field strength between the center and the planes of the coils. A slightly larger value of reduces the difference in field between the center and the planes of the coils, at the expense of worsening the field's uniformity in the region near the center, as measured by .
The calculation of the exact magnetic field at any point in space is mathematically complex and involves the study of Bessel functions. Things are simpler along the axis of the coil-pair, and it is convenient to think about the Taylor series expansion of the field strength as a function of , the distance from the central point of the coil-pair along the axis. By symmetry, the odd-order terms in the expansion are zero. By arranging the coils so that the origin is an inflection point for the field strength due to each coil separately, one can guarantee that the order term is also zero, and hence the leading non-constant term is of order . The inflection point for a simple coil is located along the coil axis at a distance from its centre. Thus the locations for the two coils are .
The calculation detailed below gives the exact value of the magnetic field at the center point. If the radius is R, the number of turns in each coil is n and the current through the coils is I, then the magnetic flux density B at the midpoint between the coils will be given by
where is the permeability of free space ().
- = the permeability constant =
- = coil current, in amperes,
- = coil radius, in meters,
- = coil distance, on axis, to point, in meters.
The Helmholtz coils consists of n turns of wire, so the equivalent current in a one-turn coil is n times the current I in the n-turn coil. Substituting nI for I in the above formula gives the field for an n-turn coil:
In a Helmholtz coil, a point halfway between the two loops has an x value equal to R/2, so calculate the field strength at that point:
There are also two coils instead of one (the coil above is at x=0; there is a second coil at x=R). From symmetry, the field strength at the midpoint will be twice the single coil value:
To improve the uniformity of the field in the space inside the coils, additional coils can be added around the outside. James Clerk Maxwell showed in 1873 that a third larger-diameter coil located midway between the two Helmholtz coils can reduce the variance of the field on the axis to zero up to the sixth derivative of position. This is sometimes called a Maxwell coil.
- Maxwell coil
- Halbach array
- A magnetic bottle has the same structure as Helmholtz coils, but with the magnets separated further apart so that the field expands in the middle, trapping charged particles with the diverging field lines. If one coil is reversed, it produces a cusp trap, which also traps charged particles.
- Ramsden, Edward (2006). Hall-effect sensors : theory and applications (2nd ed. ed.). Amsterdam: Elsevier/Newnes. p. 195. ISBN 978-0-75067934-3.
- Helmholtz Coil in CGS unitsArchived March 24, 2012 at the Wayback Machine
- "Earth Field Magnetometer: Helmholtz coil"[dead link] by Richard Wotiz 2004
|Wikimedia Commons has media related to Helmholtz coils.|
- On-Axis Field of an Ideal Helmholtz Coil
- Axial field of a real Helmholtz coil pair
- Helmholtz-Coil Fields by Franz Kraft, The Wolfram Demonstrations Project.
- Kevin Kuns (2007) Calculation of Magnetic Field inside Plasma Chamber, uses elliptic integrals and their derivatives to compute off-axis fields, from PBworks.
- DeTroye, David J.; Chase, Ronald J. (November 1994), The Calculation and Measurement of Helmholtz Coil Fields (PDF), Army Research Laboratory, ARL-TN-35
- Magnetic Fields of Coils