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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