Magnetism, the invisible force that guides compasses and powers electric motors, is fundamentally tied to specific units of magnetism that quantify its behavior. These units provide the language for engineers, physicists, and technicians to describe everything from the strength of a permanent magnet to the subtle fluctuations in the Earth’s magnetic field.
Core Magnetic Quantities and Their Units
To understand units of magnetism, one must first distinguish between the source of a magnetic field and its effects. The magnetic field strength, denoted as H , describes the magnetizing force applied to a material. In the International System of Units (SI), this is measured in amperes per meter (A/m). In the centimeter-gram-second (CGS) system, the equivalent unit is the oersted (Oe), where 1 Oe is approximately equal to 79.577 A/m.
The Tesla and Magnetic Flux Density
While field strength describes the source, magnetic flux density (denoted B ) describes the actual magnetic field passing through a specific area, including the material's response. This is the quantity most people associate with the "strength" of a magnet. The SI unit for magnetic flux density is the tesla (T), named after inventor Nikola Tesla. A tesla is a large unit; a typical refrigerator magnet has a flux density of roughly 0.001 T, or 1 millitesla (mT).
Converting to CGS: The Gauss
In the CGS system, the unit for magnetic flux density is the gauss (G), named after Carl Friedrich Gauss. The relationship between these units is precise: 1 tesla equals 10,000 gauss. While the tesla dominates modern scientific and engineering documentation, the gauss remains prevalent in specific applications such as magnetic therapy, older scientific equipment, and the assessment of magnetic shielding effectiveness.
Magnetic Flux: The Total Flow
Units of magnetism also account for the total amount of magnetic field passing through a surface, known as magnetic flux. This is represented by the Greek letter phi (Φ). The SI unit for magnetic flux is the weber (Wb), where one weber is equivalent to one tesla meter squared (T·m²). In CGS, the unit is the maxwell (Mx), and the conversion is similarly vast: 1 weber equals 100,000,000 maxwells. This unit is critical for designing transformers, inductors, and understanding electromagnetic induction.
Intrinsic Properties: The Demagnetization Curve
For manufacturers and material scientists, the most relevant units of magnetism describe the inherent properties of the material itself. The remanence (denoted B r ) is the magnetic flux density remaining in a magnet after the external magnetizing field is removed. It is measured in tesla (T) or gauss (G). Another critical metric is the coercivity (H c ), which measures a material's resistance to becoming demagnetized. This is expressed in units of oersteds (Oe) in CGS or amperes per meter (A/m) in SI.
The Joule and Magnetostatic Energy
The energy stored in a magnetic field is quantified using units derived from the joule. The energy density—the energy per unit volume—within a magnetic field is measured in joules per cubic meter (J/m³) in SI, or gauss-oersteds (G·Oe) in CGS. This unit is particularly important for evaluating the performance of permanent magnets, as it indicates how much energy can be stored and utilized before the magnet saturates or loses its properties.
