Rheostats and Other Variable Resistances
A. Rheostats

   Many modern electrical measurement techniques can be found in Charles Wheatstone's 1843 Bakerian Lecture, "An account of several new Instruments and Processes for determining the constants of a Voltaic Circuit."
   In his paper, Wheatstone makes full use of the prefix rheo, from the Greek root meaning "to flow". A rheomotor is a source of electric current, a rheoscope is a device to device to measure current, and a rheostat is a device for maintaining a constant current. 

   At the right are three rheostats. To measure an unknown resistance, Wheatstone proposed that it be placed in series with a rheoscope (ammeter) and a rheomotor (source of EMF). The reading of the rheoscope was noted, and a rheostat inserted into the circuit in place of the unknown. The rheostat was adjusted to give the same current reading. The rheostat had previously been calibrated using Wheatstone's Bridge

   The upper picture shows an original Wheatstone-type rheostat by White of Glasgow, which is at Miami University in Oxford, Ohio. The near cylinder is made of wood with a shallow helical groove cut in it. As the cylinder is rotated, resistance wire unwinds from the wooden cylinder and winds up on the brass cylinder at the rear. 

   The middle picture shows Wheatstone's other form of rheostat, in which a single wooden cylinder with a helical groove onto which a wire of uniform cross-section is permanently wound. As the cylinder is turned, a sliding contact changes the effective length of the wire. This is the same construction used in modern multiple-turn potentiometers. The instrument was on display at the science museum of Birr Castle in Ireland in the Fall of 1999. 

   The instrument at the bottom is a rheostat by Elliott Brothers of London, on display at Vanderbilt University as part of the Garland Collection of Classical Physics Apparatus. It has 83 turns of brass wire wound on an insulating cylinder 9.5 cm in diameter. The mechanism is the same as the rheostat above it.

REFERENCE: Thomas B. Greenslade, Jr., "Nineteenth Century Textbook Illustrations XXIII -- The Rheostat", Phys. Teach., 16, 301-2 (1978)

B. Variable Resistances

   Other variable resistances tend to be black boxes, with plugs or dials on top to allow various resistance values to be selected. Here I have chosen four atypical variable resistors. 

   At the right is a resistance box, at Grinnell College in Iowa, made by A.P. Gage of Boston. Gage, who held a doctorate, was an instructor in physics at the English High School in Boston, wrote several physics textbooks, and manufactured apparatus. 

   Later the similar piece of apparatus below came into my possession and was added to the Greenslade collection. It is by W.A. Olmsted of Chicago, and differs only in having a black vulcanite top. Now I was able to take off the top and observe the resistances, which go in steps of 1 ohm, 10 ohms and 100 ohms. 


   The apparatus was also made by the Chicago Apparatus Company, and the 1896 catalogue notes that this is a "Switch Resistance Box, in neat hardwood box, with vulcanite top. The coils ... are made from German silver wire [a copper-nickel alloy] and carefully tested ... $5.00).
   Below are front and back views of a five decade variable resistor made by Max Kohl of Chemnitz. The 0.1 and 1 ohm resistors are slide wires along the front and back of the top, and the three upper ranges use coils of low-temperature coefficient resistance wire (probably constantan). It is at Washington and Lee University in Lexington, Virginia.

   Denison University bought this Leeds and Northrup Standard Low Resistance apparatus for $200.00 in January 1905. It uses two heavy manganin bars; the lengths of the slenderer bar (along the back of the apparatus) can be chosen with plugs to give steps of 0.001 Ohm, while the larger bar across the front has a slider which allows reading to 0.000001 Ohm. The apparatus can carry 50 Amperes. 

   Here is a second example of the Leeds and Northrup Kelvin Bridge, catalogue number 4300, listed at $200.00 for the Variable Standard Low Resistance unit in the 1907 catalogue. It is a solid piece of apparatus (I picked it up) and is at the physics department at Swarthmore College in Pennsylvania.

   This resistance box, made by E. S. Greeley & Co. in New York City, has unusual markings: 1/9, 1/99 and 1/999, all presumably in ohms. 

   It is in the collection of the University of Vermont. 


   The unmarked resistance box at the right is in the collection of Westminster College in New Wilmington, Pennsylvania. The steps are .1, .2, .3, .4, 1, 2, 3, 4, 10, 20, 30, 40 and 50 ohms.

   At the right is an apparatus at Dartmouth that I think is a variable liquid resistance. The wooden rod to which the black knob is attached pulls out, decreasing the amount of contact wire immersed in the liquids in the vertical columns. It bears no maker's name and is 29 cm in height.

   Physics departments used to be heavily equipped with tubular rheostats, used to pick off a certain fraction of an EMF or, more commonly, as a heavy duty variable resistor to limit current. This one is by James G. Biddle of Philadelphia, and is rated at 23 ohms and 5 A. Since the tube is made of thick ceramic, this is a conservative current rating. 

   This particular model was given to the Greenslade Collection by Daniel Chaucer.

   The two three-dial resistance boxes below were made by Leeds and Northrup and are in the Greenslade Collection. The one on the left is marked "Patent Applied For" and the right-hand one has the label "Patented July 28, 1914". Several early L&N pieces in my collection bear the same patent date.


   The  Kohlrausch Slide Wire is used as a variable resistance when making resistance measurements using bridge techniques. The form shown in the left-hand picture, below, appears in the 1911 Leeds and Northrup catalogue No. 44 ("Resistance Boxes and Wheatstone Bridges") at $60. "The bridge wire is mounted upon a marble cylinder 15 cm in diameter. There are ten turns of wire, giving a total length of 470 cm... The position of the contact is read by means of the vertical glass scale... It is easily possible to estimate to thousandths of one complete resolution, one ten-thousandths of the total motion of the contact point." The total resistance is about 7 ohms.
   In the 1927 catalogue the Slide Wire is up to $100, with a bakelite drum replacing the marble cylinder. The Students' Circular Slide Wire at the right, below, has been added at a cost of $50. End coils have been added to extend the range of the slide wire. Both of these instruments are in the Greenslade Collection.

   The small, plug-type resistance box at the left is a piece of Milvay apparatus, and was made by the Chicago Apparatus Company. Its arc-shaped arrangement of resistance coils is unique. The resistance values run from 0.1 to 2ll ohms, with a stated precision of 1/5%. In the 1929 catalogue it is listed at $14; for $2 more you could get the model with glass sides to allow students to see the coils. 

   It is in the Greenslade Collection.

   The two resistance boxes below were made by Hartmann and Braun of Berlin. The box on the left has setments marked 1000, 2000, 3000 and 4000 ohms; the one on the left has 1, 10, 10, 20, 30 and 40 ohms, and is marked ""Richtig bei 20ºC". Both are in the Greenslade Collection

   The resistance box at the right was made by Hartmann & Braun of Frankfurt am Main, Germany, and imported by Biddle of Philadelphia. It is marked "richtig bei 18 grad Celsius". 

   The two halves are identical, and have resistances of 1000, 100, 10 and 1 ohms. This curious arrangement of values suggests that this might have been used as the two ratio arms of a Wheatstone bridge. The tapered connection plugs extending to the left may be inserted in each of the brass connection blocks. 

   The apparatus is in the Greenslade Collection.

   The large Leeds & Northrup resistance box at the right, with only two widely-spaced decades, was a puzzlement until I found it in an 1907 L&N catalogue. It is a Type 5652 Curtis Coil Resistance Box "for precise measurements with high frequencies and alternating currents." The coils were designed by Curtis and Grover of the Bureau of Standards to avoid inductive effects. It's resistances have a precision of 0.04%, and it cost $250. 

   The apparatus is in the Greenslade Collection.

   I wish that I had dusted this nice resistance box before I photographed it at Case Western Reserve University in Cleveland, Ohio in October 2004. 

   It is by Fritz Kohler of Leipzig in Germany (where Max Kohl apparatus was produced) and uses manganin instead of constantan resistance wire. It was calibrated for use at 15°C, rather too chilly for modern experimenters.

   The resistance box at the right was imported by the Philadelphia firm of James W. Queen & Co. about 1900, and sold to Kenyon College. It is probably German, and the resistance coils were made of German silver (a copper-nickel alloy) wire. The original cost was about $60.

  The Leeds & Northrup three-dial resistance box at the left is included because of its age. The serial number is 860; the company was founded in 1903 and by 1926 the serial numbers were in the 150,000 range. Clearly one cannot draw a linear graph of serial number as a function of time, but it is probably true that the box is ca. 1905. 

   It is in the apparatus collection of Antioch University in Yellow Springs, Ohio.


   This small resistance box is included because it is early Central Scientific. The patent date on it is May 7, 1907, and an examination of the 1920 Cenco catalogue shows that the sheet brass resistance plug sockets had already been replaced by the more usual heavy brass bars. 

   The device is in the Greenslade Collection,

   Finally, for the traditionalists, one big black Leeds and Northup resistance box is included. This five dial box is listed at $100.00 in the 1907 catalogue. The iron sides allow the free radiation of heat when the box is air cooled. The perforations in the sides enable the coils to be immersed in oil to increase the current carrying capacity "very materially". 

   The apparatus is at Denison University in Granville, Ohio.

Return to Electrical Measurements Home Page | Return to Home Page