Helmholtz Coils
A Helmholtz coil is actually a pair of specially constructed coils mounted a fixed distance apart on a common base. Current passed through the coils produces an extremely uniform magnetic field in the space between them. For the measurement of magnets, the coils are used in a different manner – they are connected to an integrating fluxmeter, which gives an accurate indication of the overall strength of the magnet when it is withdrawn or rotated a half-turn. If a gaussmeter is used for this purpose, many measurements may have to be made because of local variations of magnetic strength. A Helmholtz coil, on the other hand, measures the entire magnet at once, in a fast, reliable, and easy manner. For this reason, Helmholtz coils are ideal for quality checking of magnet parts after they have been magnetized.
Helmholtz coils can be used to measure bar and slab-type magnets (of two poles only), but can also measure arc segments by using a correction factor. These devices operate on changes of magnetic flux only, and so are unaffected by fixed stray fields, such as the Earth’s magnetic field.
A Helmholtz coil consists of a pair of similar coils, each with a small cross-section through the wire compared to its overall diameter. The coils are mounted on a common axis, at a fixed distance apart (at or about half the average coil diameter). A number of variations exist, including use of rectangular coils, and other numbers of coils than two. In a two-coil Helmholtz pair connected in series, if electric current is passed through them, a very uniform magnetic field is generated in the space between the coils. At the center, the first, second, and third derivatives of the field all vanish. Helmholtz coils are often used to measure magnets, rather than to create a field. In order to do this, the coil pair is connected to a fluxmeter, which integrates voltage over time.
By Faraday’s law:
And so:
If a magnet is placed in the space between the coils, aligned with the coil axis, and the meter is zeroed, then when the magnet is withdrawn from the coil (regardless of the rate), the fluxmeter indication is proportional to the magnetic moment of the magnet. The magnet’s magnetic moment may be defined either as the average axial magnetic flux density in the magnet, times its volume; or as the magnetic pole strength (an amount of flux) times the magnetic pole spacing. The fluxmeter/Helmoltz coil combination may be very fast, accurate and easy to use for bar magnets, using proper equipment. With a suitable correction, it may also be used for measuring arc segments. This method will not work for magnets with multiple pole pairs, or for ring mangets with radial magnetization, however.