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A system with two Helmholtz coils is a device for producing a large volume of almost constant magnetic field. It is named in honor of the German physicist Hermann von Helmholtz.
Two identical circular magnetic coils are placed on one side and on the opposite side of the experimental area. The two coils are put parallel one to the other and separated by a distance equal to the radius of the coil.
In the two coils an equal electrical current flowing in the same direction.
In the central part of the experimental zone a magnetic field is produced that is costant in time and is with smooth difference in the neighbour places. This apparatus create a magnetic field with small gradient of the magnetic field against the space.
The calculus of the exact magnetic field has mathematical complexities and involves the study of Bessel functions. An approximation calculus (with give the right value for the centre point) is easier.
If the radius, R, the number of turns in each coil, n and the current flowing through the coils, I, then the magnetic field, B at the midpoint between the coils will be given by
- <math> B = {\left ( \frac{4}{5} \right )}^{3/2} \frac{\mu_0 n I}{R}.<math>
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