In a manner relating to or using galvanometry; by means of measuring electric current with a galvanometer.
Galvanometrical + -ly (adverb-forming suffix). This creates an adverb describing how something is done using galvanometric methods or principles.
When scientists wrote 'the current was galvanometrically measured,' they were announcing that they used the most reliable, objective method available at that time—it was basically the scientific equivalent of saying 'we have proof.'
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