CGS e.m. unit to Weber per henry

The CGS e.m. unit of current (10 A) was inconveniently large for everyday lab work, while the CGS e.m. unit of resistance (the abohm, 10⁻⁹ Ω) was absurdly small. Physicists created "practical" units — the ampere, volt, and ohm — as decimal multiples that gave human-scale numbers. The ampere was set at 0.1 abampere. These practical units eventually became SI, while the "absolute" CGS units became historical footnotes.

In the 19th century, electricity and magnetism were treated as partially separate phenomena, leading to separate "natural" unit choices. The EMU system normalized magnetic permeability to 1; the ESU system normalized electric permittivity to 1; the Gaussian system mixed both. Once Maxwell unified electromagnetism, this fragmentation became unnecessary — but the systems persisted in literature for a century.

They introduced "practical" units — the ampere, volt, and ohm — as decimal multiples of CGS-EMU quantities. The ampere was defined as 0.1 abampere (CGS e.m. unit). This practical system eventually became SI, while the "absolute" CGS units faded. The factor of 10 was chosen for human-scale convenience.

The gauss (magnetic flux density, = 10⁻⁴ tesla) remains surprisingly common — refrigerator magnets are rated in gauss, and MRI field strengths are often quoted in both tesla and gauss. The oersted (magnetic field strength) appears in materials science. These CGS-EMU holdouts persist because their numerical values are more convenient for everyday magnets.

The SI was officially adopted in 1960, but the transition took decades. Most physics journals required SI by the 1970s, though astrophysics and plasma physics held onto Gaussian CGS into the 2000s. Some subfields never fully switched — you can still find new papers using gauss and oersted alongside tesla and A/m.

When designing a transformer, you start with the required flux (webers) to transfer power at a given voltage and frequency. The core's inductance (henries) is set by geometry and material. Dividing flux by inductance gives the magnetising current that must flow — and if it is too high, the core saturates and the transformer overheats.

One weber is the magnetic flux that, when reduced to zero in one second, induces one volt in a single-turn coil. A small transformer core might carry 0.001 Wb (1 mWb) of peak flux. The Earth's magnetic field through a 1 m² loop is about 50 μWb. One weber is actually an enormous amount of flux in everyday terms.

If the calculated magnetising current (Wb/H) exceeds design limits, the core is approaching magnetic saturation. The inductance drops sharply, current spikes further, and the inductor or transformer overheats. Solutions include using a larger core, higher-permeability material, an air gap, or reducing the operating flux density.

Every magnetic core has a saturation flux density (e.g., 1.5 T for silicon steel, 0.3 T for ferrite). When flux approaches this limit, permeability collapses, inductance plummets, and Wb/H (current) shoots up. Power supply designers must ensure peak flux stays 20–30% below saturation under worst-case conditions.

An air gap dramatically increases the reluctance of the magnetic circuit, which lowers inductance (H) for the same core geometry. For a given flux (Wb), the magnetising current (Wb/H) increases — but the core is far harder to saturate. Power supply designers deliberately add 0.1–1 mm air gaps to ferrite cores so the inductor can handle higher peak currents without the flux density hitting saturation limits.