Formula: Helmholtz Coil (Same Current Direction) Magnetic flux density (B-field) Electric current Number of turns
$$\class{violet}{B}(z) = \frac{\mu_0 \, I \, R^2 \, N}{2} \, \left[ \left( (z-d/2)^2 + R^2 \right)^{-3/2} + \left( (z+d/2)^2 + R^2 \right)^{-3/2} \right]$$
$$\class{violet}{B}(z) = \frac{\mu_0 \, I \, R^2 \, N}{2} \, \left[ \left( (z-d/2)^2 + R^2 \right)^{-3/2} + \left( (z+d/2)^2 + R^2 \right)^{-3/2} \right]$$
Magnetic flux density (B-field)
$$ \class{violet}{B} $$ Unit $$ \mathrm{T} $$
Magnetic field along the symmetry axis (here along the \(z\) coordinate). For the magnetic field between the coils to be reasonably homogeneous, the distance \(d\) and the radius \(R\) must be equal.
Field point
$$ z $$ Unit $$ \mathrm{m} $$
Spatial coordinate \(z\) indicates the location where the magnetic field \(B(z)\) is present.
Distance
$$ d $$ Unit $$ \mathrm{m} $$
Distance between both coils.
Electric current
$$ \class{red}{\boldsymbol I} $$ Unit $$ \mathrm{A} = \frac{ \mathrm C }{ \mathrm s } $$
Electric current along a Helmholtz coil. The current in both coils is equal in magnitude. For this formula to work, the current in both coils must circle in the same direction.
Number of turns
$$ N $$ Unit $$ - $$
Coil windings. With \(N\) windings, the current goes \(N\) times along the coil.
Vacuum permeability
$$ \mu_0 $$ Unit $$ \frac{\mathrm{Vs}}{\mathrm{Am}} = \frac{ \mathrm{kg} \, \mathrm{m} }{ \mathrm{A}^2 \, \mathrm{s}^2 } $$
The vacuum permeability is a physical constant and has the following experimentally determined value:
$$ \mu_0 ~=~ 1.256 \, 637 \, 062 \, 12 ~\cdot~ 10^{-6} \, \frac{\mathrm{Vs}}{\mathrm{Am}} $$