Leeds & Northrup Light Beam Galvanometer, Model 2420, Serial #160826featuring an integrated No. 2194 light source assembly, served as an ultra-sensitive scientific laboratory tool to detect and measure minuscule electrical currents, bounces a focused beam of light off a tiny internal mirror suspended on a delicate wire ribbon, when electrical current passes through the internal coil, the suspension twists, turning the mirror, deflects the light beam across the translucent scale at the front, indicating a current charge, outer wood case in good condition, instruction paper inside the lid completely intact, normal age-related amber yellowing, but clean and crack-free, minor oxidation on hardware, good condition, see photos. 8 1/4" x 4 1/4" x 5 1/4"