Magnetic Field Strength Converter

Magnetic field strength, denoted H, is one of two fundamental field quantities used to describe magnetic phenomena. It represents the magnetic field produced exclusively by free electric currents — the currents in conductors — without including the contribution from the magnetization of any material present. The SI unit is ampere per meter (A/m). Magnetic field strength H is related to magnetic flux density B and material magnetization M by B = μ₀(H + M) in SI units, where μ₀ = 4π × 10⁻⁷ H/m is the permeability of free space. In simpler terms for linear media: B = μ₀μᵣH, where μᵣ is the relative permeability of the material.

The distinction between H and B is crucial for material characterization. When engineers specify the performance of a permanent magnet, they describe the B-H demagnetization curve — a plot of magnetic flux density B versus applied opposing field H. The coercive field Hc (in A/m or kA/m) is the field strength at which the net flux density returns to zero, indicating how resistant the magnet is to demagnetization. Rare-earth permanent magnets like neodymium-iron-boron (NdFeB) achieve intrinsic coercivities of 1000–3000 kA/m, making them far superior to older alnico or ferrite magnets for applications requiring field stability under mechanical or thermal stress.

The oersted (Oe) is the CGS unit of magnetic field strength, named after Hans Christian Ørsted who discovered electromagnetic induction in 1820. 1 Oe = 1000/(4π) A/m ≈ 79.5775 A/m. The oersted remains common in US and Japanese magnetic industry specifications. Hard disk drive heads and magnetic recording media specifications routinely cite coercivity in oersteds — a typical modern hard drive media has coercivity of 3000–5000 Oe. When working with legacy specifications or data from US suppliers, converting oersteds to A/m (for SI calculations) is a daily task for magnetic engineers.

In transformer and inductor core design, the B-H curve of the core material determines the inductance as a function of current. Silicon-iron laminations saturate at approximately H = 10,000 A/m (B ≈ 2 T). Ferrite cores for high-frequency switching power supplies typically saturate at H = 200–1000 A/m. Knowing the H field in A/m for a given excitation current allows engineers to determine the operating point on the B-H curve and ensure the design remains in the linear (non-saturated) region for normal operation.

In MRI and medical imaging, the static main field B₀ is often specified in tesla, but gradient fields and RF excitation fields are analyzed in terms of H. Specific absorption rate (SAR) calculations in patients involve the H field distribution inside human tissue, as the electromagnetic power deposition relates to σ|H|² (where σ is tissue conductivity). Regulatory limits for RF exposure in MRI are specified as limits on incident H field strength at the patient's surface.

In electromagnetic compatibility (EMC) testing, magnetic field immunity tests (IEC 61000-4-8) expose equipment to power-frequency magnetic fields of 1 A/m to 100 A/m or higher. Geomagnetic field strength at Earth's surface is approximately 25–65 A/m (the Earth's B field of 25–65 µT divided by μ₀). Sensitive scientific instruments and precision electronics must be designed to operate despite these ambient field strengths.

In non-destructive testing (NDT) using magnetic particle inspection (MPI), the surface field strength needed to detect surface cracks is typically 2–6 kA/m. Magnetic flux leakage (MFL) testing of pipelines uses permanent magnets or electromagnets producing H fields of 50–400 kA/m in the pipe wall to detect corrosion-induced wall loss. Specifications for magnetization levels in MPI standards (ASTM E1444, EN ISO 9934) are given in A/m and kA/m.

In magnetic resonance and spectroscopy research, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectrometers specify their static magnetic field in tesla, but the sweep field, modulation field, and microwave B₁ field are often analyzed in A/m or mA/m. The kiloampere/meter unit is convenient for expressing the large H fields in high-field research magnets.

This magnetic field strength converter supports all 4 units: ampere/meter (SI), ampere turn/meter (numerically equivalent), kiloampere/meter, and oersted (CGS) — instantly and precisely to 12 significant digits, completely free.