Of or pertaining to galvanometry; relating to the measurement of electric current using a galvanometer.
Galvanometer + -ical (variant adjectival suffix). The -ical ending is a less common variant of -ic, often used in older technical texts to create adjectives with a slightly more formal tone.
You'll mostly find 'galvanometrical' in older scientific texts from the Victorian era—it's the fancier, more formal way to say 'galvanometric,' showing how scientific language loved to add extra suffixes for sophistication.
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