Galvanometer
2 of the 6 encyclopedias on this shelf carry an entry for Galvanometer. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
an instrument used to indicate the presence, the direction, or the strength of an electric current. If a wire or other conductor through which a current of electricity is passing be brought near a freely suspended magnetic needle , the latter betrays a tendency to swing around and place itself in a position at right angles to the path of the passing current. The amount of the deflection of the needle from a north and south direction is proportioned to the strength of the current. If the wire through which the current is passing be coiled back and forth, the deflection is proportionate to the number of the turns of the wire. By giving the wire many turns it is possible to discover the presence, as well as the direction, of a very small current. Such an instrument is called a galvanoscope. If the needle be provided with a dial, and the dial be marked with figures to indicate the number of degrees of deflection, the instrument is called a galvanometer. There are several models, known as the tangent, the astatic, and the d'Arsonval. The facts on which the galvanometer rests were discovered by a physicist named Oersted.
Collier's New Encyclopedia (1921)
an instrument for detecting the presence and measuring the intensity of feeble galvanic currents. A differential galvanometer is an instrument designed to ascertain a difference in the intensity of two galvanic currents. A marine galvanometer is a galvanometer designed to test the insulation of submarine cables, and at the same time unaffected by the pitching and rolling of the ship. GALVESTON SEA WALL
What Has Changed in the Last Century
The physics is exactly as Aiton's described in 1910, and the credit to Oersted, who noticed a current deflecting a compass needle in 1820, still stands. What has gone is the instrument's central place in electrical work: the sensitive suspended-needle and mirror types that Collier's still treated as standard in 1921, including the marine galvanometer for testing submarine cables aboard a rolling ship, were displaced first by rugged moving-coil pointer meters descended from the d'Arsonval pattern and then, from the 1970s onward, by digital multimeters with electronic amplifiers and no moving parts at all. The name survives in two odd corners: the analogue needle meter still called a galvanometer in teaching labs, and the galvanometer scanner, a tiny mirror on a coil that steers laser beams in engravers, projectors and barcode readers. So the entries describe a working principle that is still taught and a workshop instrument that has largely vanished.
Written for this edition, 2026.