VIM concept: influence quantities — measuring one thing while another moves

The VIM's influence quantity (2.52) is the concept behind most mysterious measurement disagreements: a quantity that isn't the thing you're measuring, doesn't change the thing you're measuring, but changes the RELATION between your indication and the result.

Temperature is the canonical one — the micrometer reads length, but its frame and the part expand differently as the room drifts. Line voltage sags, barometric pressure moves a balance's buoyancy, RF fields ride into thermocouple leads. None of these touch the measurand; all of them touch the reading.

The discipline the definition enforces: identify the influence quantities, then either control them (the rated operating conditions of VIM 4.9), correct for them (with the correction's own uncertainty budgeted), or bound them as a Type B component. GMP 10 in the library is essentially an influence-quantity census for weighing — drafts, vibration, thermal gradients, operator heat — and a model for writing the same census for any bench.