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to an integrating fluxmeter, which gives an accurate indication of the overall strength of the magnet when it is withdrawn or rotated a half-turn.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.