The Potentiometer

   The slide-wire potentiometer was invented by Johann Christian Poggendorff (1796-1877) in 1841. In addition to his work with electricity, he was the editor of the Annalen de Physik und Chemie from 1824 to 1876, and his biographical volumes of scientists have become standards and are still being published.

   The potentiometer is a Leeds and Northrup Type K model, which was a standard piece of apparatus in most college and university electrical measurements laboratories for the first half of the 20th century. By the time I used one in the 1950s the model designation was the Type K-3, but the basic design remained the same. This particular instrument was used at Denison University.

   At the heart of the potentiometer is a long slide wire. In the Type K potentiometer this was in the form of a ten-turn coil, a design due to Kohlrausch. The potentiometer was used with an external standard cell and an external galvanometer. Using a potentiometer to make voltage measurements was somewhat slow, but you could read to 0.00001 Volt.

   The similar Type-K potentiometer below is in the Greenslade Collection. It is listed in the 1922 Leeds and Northrup catalogue at $275. This is perhaps one eighth of a typical annual faculty salary at the time.

An older form of potentiometer, with all settings switch-selected is this Otto Wolff (Berlin) potentiometer that was given to the Greenslade Collection by Daniel Chaucer. It is listed in the 1900 Wolff catalogue at 600 Marks, about $150. 

   Leeds and Northrup made a series of lesser potentiometers designed for introductory work by students. This student potentiometer has a single-turn slide wire instead of the ten-turn slide wire used in the Type-K series. 

   This particular instrument is in the Greenslade Collection.


   This somewhat larger and newer version of the Leeds & Northrup student potentiometer above sold in 1940 for $70. 

   It was designed for use with a Weston-type standard cell with an EMF of 1.018 V; under this condition it was direct-reading. Its slide wire and fifteen stepwise resistors could be used as two arms of a Wheatstone bridge. 

   It is in the Greenslade Collection.

   From the 1940 Central Scientific Company catalogue: “Volt Box, L&N, for use with Students’ Potentiometer and Type K Potentiometer. The volt box has three ranges which permit measurement and calibration up to 15, 150 and 300 volts, respectively. Accuracy 0.04%. In polished mahogany case, with insulated binding posts and resistance coils attached to polished black top. The case is protected with ventilating holes so that the indicated voltages may be used without overheating the resistance coils … $55.00”

   This instrument, in the Greenslade Collection, has a tag that says that it was made by Leeds and Northrup in 1927. It has always had its protective mahogany lid in place, and looks brand new. 

   The volt box on the left was made by James W. Queen of Philadelphia. It looks as if it has undergone some internal modifications. The paper labels notes that it will divide the input voltage by 200, 100 or 10. 

    Daniel Chaucer gave this instrument to the Greenslade Collection.

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Specially-designed potentiometers were also used for temperature measurements using thermocouples as sensors.