# Encyclopedia Magnetica

### Site Tools

calculator:helmholtz_coil

### Calculator of H at the centre of a Helmholtz coil

This section is an interactive calculator.
Magnetic field strength H at the centre of a Helmholtz coil
$$H_{centre}(t) = \frac{N_{each}·I (t)· 8 } {r·\sqrt{125}} \approx 0.71554·\frac{N_{each}·I}{r}$$ (A/m)
where: $I(t)$ - current (A) at time $t$ (s), $N_{each}$ - number of turns (unitless) of each half-coil, $r$ - radius (m)
 Current I = (nA) (μA) (mA) (A)      Radius r = (mm) (cm) (inch) (m) Choose option: turns per half-coil turns total      Number of turns N = (unitless) H = (A/m) (kA/m) (A/cm)        B (μ0) = (T) (mT) (uT) Notes: This equation is valid only for infinitely “thin” coils. The instantaneous values of H are directly proportional to the instantaneous values of I. The value of B(μ0) is for vacuum.