Maxwell and Thomson defined the ohm as a repeatable experiment

By the early 1860s, transatlantic telegraph cables were failing or underperforming in ways engineers struggled to diagnose, because signal speed depended directly on a cable's electrical resistance and nobody had a reliable way to say what "one unit of resistance" actually meant. Telegraph companies and instrument makers each worked from their own ad hoc reference — typically a specific length of a specific wire kept in their own workshop — so a resistance measurement made in one lab could not be trusted or even compared against one made in another.

The obvious fix, and the one German instrument maker Werner Siemens proposed to the same 1861 British Association committee, was to settle on one universally agreed physical object as the reference — in his case, a standard column of mercury of specified dimensions — and have every laboratory in the world calibrate against copies of it.

Defined the ohm as a repeatable experiment—a coil spun in a known field—so any lab could derive the unit from scratch, no master artifact needed.

A procedure-based definition is independent of any single artifact's condition.

Any lab with the right equipment can derive the same unit from scratch.

Physical copies can be re-verified against the experiment if they drift.

Define standards as repeatable procedures grounded in stable laws, not as physical artifacts, so anyone can reproduce them independently.

The committee still fabricated a set of physical wire-coil resistors in 1865 as practical everyday references for labs to work with, but those objects were calibrated against — and could always be re-verified against — the reproducible spinning-coil experiment, rather than being the definition themselves. The BA's units of resistance, current and voltage became essentially the ohm, amp and volt still in use today, with far-reaching effect on all later precision electrical measurement.