Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Simplified (Student) Potentiometer
Leeds & Northrup Co., Philadelphia
Humboldt S-T-C; c.1930
The potentiometer is used to measure voltages by comparison to a known reference voltage. The current instrument is composed of a series of precision resistors which may be placed in series with a precision slide-wire resistor to create a resistance of known value. In use the instrument is connected to known and unknown voltage sources, and the resistors are adjusted until the voltages are matched as indicated by a null reading on a sensitive galvanometer. The instrument is also set-up so that the known source may be readily calibrated against a reference voltage source (generally a Weston standard cell). The potentiometer made up the heart of a great number of instruments for measuring such quantities as pH, conductivity, light intensity etc.
The selector switch move a leaf spring along a heavy metal ring, alternately connecting to 16 metal disks to give ranges of 0-1.5 in 0.1 increments. Adjacent to it is the potentiometer dial with scales of 0-1000 (in red) and 0.000-0.100 Volt (in black). There are ten binding posts, five along the left side labeled: BA+, .01, 1, R, and E-; and five along the right side labeled: L', L, E+, H, and H'. The instrument has a 7 x 12 1/2" polished Bakelite top-plate, and a polished solid mahogany case and cover. It is 4 1/8" high to the top of the bakelite plate. Condition: excellent, with some scratches noted on the cover, and small chip on right lower base of case. A serial number, 724495, is engraved on teh bakelite panel. A brass tag is nailed onto the mahogony base: Humboldt S-T-C 4879.