A Helmholtz coil is a device for producing a region of nearly uniform magnetic field. It consists of two identical circular magnetic coils that are placed symmetrically, one on each side of the experimental area along a common axis, and separated by a distance (h) equal to the radius (R) of the coil. Each coil carries an equal electrical current flowing in the same direction. A number of variations exist, including use of rectangular coils, and numbers of coils other than two. However, a two-coil Helmholtz pair is the standard model, with coils that are circular and in shape and flat on the sides. In such a device, electric current is passed through the coil for the purpose of creating a very uniform magnetic field.
Helmholtz coils are used for a variety of purposes. In one instance, they were used in an argon tube experiment to measure the charge to mass ratio (e:m)of electrons. In addition, they are often used to measure the strength and fields of permanent magnets. In order to do this, the coil pair is connected to a fluxmeter, a device which contains measuring coils and electronics that evaluate the change of voltage in the measuring coils to calculate the overall magnetic flux.In some applications, a Helmholtz coil is used to cancel out Earth’s magnetic field, producing a region with a magnetic field intensity much closer to zero. This can be used to see how electrical charges and magnetic fields operate when not acted on by the gravitational pull of the Earth or other celestial bodies.
In a Helmholtz girl, the magnetic flux density of a field generated (represented by B) can be expressed mathematically by the equation:
Where R is the radius of the coils, n is the number of turns in each coil, I is the current flowing through the coils, and ?0 is the permeability of free space (1.26 x 10-6 T • m/A).
We have written many articles about the Helmholtz Coil for Universe Today. Here’s an article about the right hand rule magnetic field, and here’s an article about magnetic field.
We’ve also recorded an entire episode of Astronomy Cast all about Magnetism. Listen here, Episode 42: Magnetism Everywhere.