The Tesla Unit
His name is a unit of measurement. Edison's isn't. How the tesla got named — and what one tesla actually means.
What it is
The tesla (symbol T) is the SI derived unit of magnetic flux density — the strength of a magnetic field. One tesla equals one weber per square metre (1 T = 1 Wb/m²): the magnetic flux of one weber passing perpendicularly through one square metre. In SI base units, 1 T = 1 kg·s⁻²·A⁻¹. It measures the B-field — the field that deflects a compass needle, steers a particle accelerator, and makes an MRI machine work.
How it got named
In 1960, the 11th General Conference on Weights and Measures (CGPM) — the body that maintains the metric system — adopted the tesla as the SI unit of magnetic flux density, naming it in honour of Nikola Tesla. The proposal came from the Slovenian electrical engineer France Avčin. Tesla had died seventeen years earlier, nearly penniless in a New York hotel; the honor arrived, as so many did, posthumously.
As with every SI unit named for a person, the convention is exact: the symbol is uppercase (T), but the name is written lowercase — “tesla,” not “Tesla” — except at the start of a sentence.
What one tesla feels like
The unit is inconveniently large for everyday life, which is why the gauss (the older cgs unit; 1 T = 10,000 G) still hangs around:
| Field | Strength |
|---|---|
| Earth’s magnetic field at the surface | ~25–65 microteslas |
| A refrigerator magnet | ~5 milliteslas |
| An MRI scanner | 1.5–3 teslas |
| The strongest sustained laboratory fields | ~45 teslas |
| The strongest pulsed fields (destructive) | ~100+ teslas |
Your compass needle moves in a field of about fifty millionths of a tesla. The unit that bears Tesla’s name is, fittingly, an inventor’s-scale unit — enormous by household standards.
The company it keeps
The tesla sits in distinguished company: the newton, the joule, the watt, the volt, the ampere, the ohm, the hertz, the farad, the henry, the weber. The roll call of the electrified world, each name a person — and Tesla is on it. Thomas Edison, for all his fame, has no SI unit. The man who lost the War of Currents won the units table.
Source notes
- Unit definition (magnetic flux density; 1 T = 1 Wb/m²; SI base-unit expression): the SI Brochure (BIPM).
- Naming in 1960 at the 11th CGPM, on the proposal of France Avčin: the CGPM proceedings and the standards literature.
- Symbol/name convention (uppercase T, lowercase “tesla”): the SI Brochure.
- Gauss relation (1 T = 10,000 G) and the field-strength examples: standard physics references; values are approximate and labeled as such.
- The “Edison has no SI unit” observation: the SI unit roster — a factual comparison, not a judgment.