A variant term for a gradiometer; an instrument that measures the gradient or rate of change of a physical quantity.
From 'grade' or 'gradient' plus Greek 'metron' (measure). This is an alternative, less common naming of the same instrument as 'gradiometer'.
Scientists sometimes use competing names for the same instrument—'gradometer' and 'gradiometer' coexist because different research communities adopted different terms, but they're measuring the exact same thing.
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