Du Bois-Reymond's compensating magnet. It allowed the one operating a galvanometer to fine-tune its sensitivity by repositioning a permanent magnet as necessary in order to compensate wholly or partially for the Earth's magnetic pull on the galvanometer needle. The wooden guide was affixed to the center of the galvanometer and could rotate freely; the magnet, in a brass mount, could be precisely positioned at any distance from the center. A pulley and string arrangement allowed the operator to adjust the magnet on the fly while seated a distance away at the reading telescope.
