The henry per meter (symbolized H/m) is the unit of magnetic permeability in the International System of Units ( SI ). Reduced to base units in SI, 1 H/m is the equivalent of one kilogram meter per second squared per ampere squared (kg times m times s -2 times A -2 ).
Permeability is a characteristic of matter, and is important in the design and construction of electromagnets, inductors, and alternating-current ( AC ) transformer s. The permeability of a vacuum (also known as free space) is equal to approximately 1.257 x 10 -6 H/m, or 1.257 microhenrys per meter (1.257 µH/m).
The henry per meter is a large unit of permeability. Virtually all substances have permeability far less than 1 (unity). However, permeability can also be expressed as the ratio of the number of henrys per meter in a particular substance to 1.257 x 10 -6 H/m. In this scheme, free space has a relative permeability of 1, and most substances have relative permeability greater than or equal to this.