What Is A Helmholtz Coil?

A Helmholtz coil, in its usual, basic configuration, consists of two similar concentrated coils of small winding cross section compared to coil radius, arranged on a single axis, at a spacing of one coil radius along their common centerline.

If electric current is passed through the coils, a very uniform magnetic field is produced in the space between them.

If a Helmholtz coil is connected as a sensor to a fluxmeter, then if a bar, plate, or arc magnet is placed in the center with the magnetic axis parallel to the coil axis, and the magnet is then removed (or rotated 180°), the resultant output of the fluxmeter can be shown to be proportional to the magnetic moment of the magnet.

The magnetic moment may be defined either as the product of the magnetic flux through the magnet\ times the pole spacing of the magnet, or as the average axial flux density of the magnet times the magnet volume:

M = φ I2 = Bav Vm (2.62)

where M = magnetic moment
φ = flux through the magnet
Iρ = pole spacing within the magnet
Bav = average flux density in the axial direction, in the magnet
Vm = magnet geometric volume

The combination of a fluxmeter and a Helmholtz coil becomes an accurate, fast, and easy way to determine the strength of a magnet with one measurement.

Although originally intended for use with bar or plate magnets, the method can also be used with arc segments (which are used in some permanent-magnet motor rotors).

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