Units of H Field: Magnetic Field Strength Explained
The unit of the magnetic field strength, usually written as H, is the ampere per metre (A/m) in the SI system. Do not confuse it with magnetic flux density, B, which is measured in teslas (T); the two quantities are related but are not the same.
What H Means
In electromagnetism, H is called the magnetic field strength or magnetic field intensity. It describes the magnetic field associated with electric currents and, in magnetic materials, helps distinguish the field produced by applied currents from the material’s magnetic response.
The SI unit A/m follows directly from the way H is defined in magnetostatics. For a long, straight wire carrying current I, the field strength at distance r in free space is H = I/(2πr). Amperes divided by metres give A/m. For example, a 1 A current at a distance of 1 m produces an idealized field strength of about 0.159 A/m.
H and B Are Different Quantities
B, magnetic flux density, describes the magnetic field in terms of magnetic flux through an area. Its SI unit is the tesla, equivalent to a weber per square metre (Wb/m²). H is a field strength, measured in A/m. They are connected by a material-dependent relationship, not by identical units.
In a simple, linear, isotropic material, the relationship is commonly written:
B = μH
Here, μ is the material’s permeability, measured in henries per metre (H/m), or equivalently N/A². Multiplying H/m by A/m gives teslas, as required for B. In vacuum, μ is the vacuum permeability, often written μ0; thus B = μ0H.
In magnetic materials, the relationship can be more complicated. A material’s magnetization can depend on its history, direction, and field strength, so a single constant permeability may not describe it accurately. In that situation, B and H remain distinct quantities even when their connection is nonlinear.
SI and Other Units
The standard SI unit for H is A/m. Another SI expression, ampere per centimetre (A/cm), is sometimes encountered in older references or specialized material data. Since one metre contains 100 centimetres:
- 1 A/cm = 100 A/m.
- 1 A/m = 0.01 A/cm.
The oersted (Oe) is a centimetre-gram-second (CGS) unit used in some older literature and magnetic specifications. In vacuum, 1 Oe corresponds to 1,000/(4π) A/m, approximately 79.577 A/m. Conversely, 1 A/m is approximately 0.01257 Oe. When converting values, check whether the source is giving H in oersteds or B in gauss: those are different CGS quantities, just as A/m and tesla are different SI quantities.
Unit names can be confusing because the symbol H is also used for the henry, the SI unit of inductance. Context matters: “H field” denotes the magnetic field-strength quantity, while “1 H” as a circuit component unit means one henry.
How H Is Determined
The appropriate expression for H depends on the geometry and the distribution of current. In a region with sufficient symmetry, Ampère’s law can be used to determine the field. For a long solenoid with N turns over length ℓ, carrying current I, the approximate field inside is:
H ≈ NI/ℓ
Here, N is the number of turns (dimensionless), I is current in amperes, and ℓ is coil length in metres, giving A/m. This ideal approximation assumes a sufficiently long, tightly wound solenoid and describes its interior field away from the ends. Real coils have fringing fields, and their geometry and surroundings can matter.
For complicated arrangements, engineers may calculate fields with numerical models or measure magnetic quantities with suitable instruments. Measurement results must identify whether the instrument reports H or B, as well as the unit and relevant measurement conditions.
Reading Values and Avoiding Mix-Ups
A field value cannot be interpreted fully from its number alone. Check the symbol, unit, location, and conditions. A stated 100 A/m is a field strength; 100 μT is a flux density. They cannot be compared directly without a relationship between B and H, including the relevant material properties.
- Keep track of distance units when using formulas. Substituting centimetres where metres are expected changes the result by a factor of 100.
- Do not treat magnetic permeability as dimensionless. Its units are needed to convert between H and B.
- Do not assume a material has constant permeability across all field strengths or magnetic histories.
- When using a CGS source, distinguish oersteds (for H) from gauss (for B).
These distinctions are useful in electromagnets, coils, magnetic materials, and electromagnetic field calculations. For a particular device or material, use the stated measurement or specification rather than converting a field value as though every medium were vacuum.
Common Misconceptions
A frequent mistake is saying that the “unit of magnetic field” is always the tesla. Tesla is the unit of B, not H. The phrase “magnetic field” is often used informally for either quantity, so technical writing should name the symbol or quantity explicitly.
Another misconception is that multiplying H by vacuum permeability always gives the correct B. That relation applies in vacuum; in materials, the appropriate constitutive relationship must account for the medium. In a simple linear material, its permeability can be used, but in nonlinear or hysteretic materials the relationship may vary with the magnetic state.
Frequently Asked Questions
What is the SI unit of H field?
The SI unit is the ampere per metre, written A/m.
Is H measured in teslas?
No. H is measured in A/m. Teslas measure magnetic flux density, B.
How are H and B related?
In vacuum, B = μ0H. In a simple linear material, B = μH; more complex magnetic materials may require a more detailed relationship.
What is the conversion from oersteds to A/m?
In vacuum, 1 Oe is approximately 79.577 A/m. Oersted is a CGS unit for H.
Is H the same as a henry?
No. H is the symbol for magnetic field strength in expressions such as A/m. The henry, also symbolized H, is a unit of inductance; the meaning is determined by context.
Related
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.