Tesla

The tesla is an SI derived unit with a special name. BIPM's SI Brochure lists it as "magnetic flux density tesla T kg s−2 A−1", so one tesla is one kilogram per second squared per ampere, equivalently one weber per square metre. It measures the density of magnetic flux: the strength of the field a watch is actually standing in.

The unit watch specifications use is the older one. NIST's Guide for the Use of the International System of Units, Special Publication 811, gives the conversion in Appendix B.9 as "gauss (Gs, G) tesla (T) 1.0 E-04": one gauss is 1 × 10⁻⁴ tesla, so one tesla is 10,000 gauss. The gauss belongs to the CGS system and is not an SI unit, which is why a figure quoted in gauss has to be divided by 10,000 to be expressed in the SI.

Anti-magnetic claims on watches are almost always stated in gauss rather than tesla. The Swiss industry has a standard for magnetic-resistant watches: FH's list of available NIHS standards names "NIHS_90-10_fr: Horlogerie - Montres resistantes au magnetisme" (1965, modified 2020), but the list gives only the title, not the text of the norm, and no source read for this entry explains why the trade kept the gauss. The persistence of the gauss is recorded here as trade convention; the conversion above is given so that either figure can be turned into the other.