Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Reflecting Galvanometer, Ballistic
Leeds & Northrup Co.
Humboldt S-T-C; c.1930
The reflecting galvanometer is a sensitive instrument for measuring current. It consists of a magnet coil with an attached mirror suspended between the poles of a magnet by a thin gold ribbon above and a coiled gold wire below. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The mirror allows the user to see very small deflections, and thus to measure very small currents, by observing a distant reflected scale (one meter or more away) through a telescope.
The ballistic galvanometer is specialized in having a wide coil characterized by a long period. It is designed to measure the quantity of electricity (charge) discharged through the instrument over a short period of time. Applications include the measurement of capacitance, inductance by comparison with a standard, and magnetic field intensities using a search coil.
L&N Type P, Ballistic, student grade moving coil reflecting galvanometer, serial number 730967. The meter consists of a 7 1/8 by 4 5/8 by 7/8" magnet covered by a bakelite plate with a center 1 3/4 by 3 3/8" window and with a hollow suspension post of nickel plated brass (4 1/4 by 3/8") on top. The L&N logo is in raised letters against a black polished region on the diamond stippled background of the bakelite cover. There are raised polished borders around the edge and the window. The mirror is 5/8" dia. There is a white backing inside the meter with the following characteristics noted (data hand entered) SENS. .0018uc/mm , C.D.R.X. 12000'~, PERIOD 27.2 Sec, RESIST 2146~, CAT. NO. 2239D. The meter is mounted on 6 1/4 by 9 by 7/8" bakelite backboard with two nickel plated brass hangers at the top, two black painted telescope arm brackets on the sides, two bakelite binding posts at the bottom on either side of a leveling screw, a manufacturers label on the bottom edge and a brass Humboldt S-T-C tag (4925) on the lower left hand corner.
The meter is mounted on a Galvanometer base, Tripod, L&N 2126, made of cast iron, identical, with the exception of the crinkle finish, to item 82315, pg. 742 of the 1960 (J-300) Cenco catalog. An essentially identical base had a decal labeled: State of California/Humboldt College/11xx1.
The meter was damaged in the Dec. 1994 Humboldt county earthquake: the brackets were bent, and the upper suspension tube was snapped off at the base where it screws into the meter frame. The brackets were subsequently straightened. However the tube is held in place by friction, and has not been repaired further.