In electromagnetism, what is the SI unit of magnetic field strength H?

The SI unit of magnetic field strength H is the ampere per metre.

Magnetic field strength, also called magnetizing field, describes the magnetizing influence produced by free electric currents and magnetized materials. Its SI unit is written A/m. The symbol H distinguishes it from magnetic flux density, which is represented by B and measured in teslas.

The distinction between H and B is one of the most common sources of confusion in electromagnetism. In a vacuum, they are related by the permeability of free space, so a field can be described using either quantity with the appropriate constant. Inside materials, magnetization changes the relationship, making the distinction especially important in engineering and physics.

The ampere per metre is not the unit of magnetic flux density. The tesla measures B, while the weber measures magnetic flux. Magnetic field strength is particularly useful when analyzing coils, solenoids, transformers, and magnetic materials.