Apparatus for Natural Philosophy:
Thomas B. Greenslade, Jr.
Kenyon College, Gambier, Ohio 43022
The development in the 19th and 20th centuries of coil wound devices to control voltage across an electrical circuit and to measure electrical current flow is examined.
In his 1843 Bakerian Lecture to the Royal Society of London, Charles Wheatstone (1802-1875) discussed a number of methods of making electrical measurements, many of which are used to this day. One of the devices he described was a rheostat.
He commented that “it is seldom that any real advance is made in a scientific theory without a corresponding change in its terminology being required”, and then defined a number of terms based on the prefix rheo, from the Greek root meaning to flow. He proposed: rheomotor, a source of electric current; rheoscope, a device to indicate the presence of a current: rheometer, a general term for a current measuring device; and rheostat; which, from its form, is clearly a device for maintaining a constant current.
A working example of Wheatstone’s rheostat is shown in Fig. 1 Wheatstone does not appear to have made any contribution to the field of electrical measurements after his Bakerian lecture, but the design of the rheostat was improved by Lord Kelvin. The instrument in the figure is in the apparatus collection of Wesleyan University in Middletown, Connecticut. The tag on it reads “Sir William Thomson’s RHEOSTAT (patent 1886) J. White, Glasgow.” James White (1824-1884) was an instrument maker in Glasgow, Scotland who worked closely with his contemporary, William Thomson, Lord Kelvin (1824-1907) in developing and then manufacturing instruments.
Wheatstone’s rheostat design depends on the observation of Georg Simon Ohm (1789-1854) that, for a wire of uniform cross section, the resistance is proportional to the length. In the example in the Bakerian lecture, one cylinder is made of wood, 6 in. long and 1.5 in. diameter, and the other is brass, and of the same size. The brass resistance wire, 0.01 in. diameter, is initially wrapped around the wooden cylinder, with the far end connected to the brass cylinder. As the crank is turned, the wire winds onto the metallic cylinder, and the effective part of the wire is thus the length remaining on the wooden cylinder. This length can be read from a scale between the cylinders and from the dial at the end of the wooden cylinder.
To find an unknown resistance, the rheostat is placed in a loop circuit in series with the galvanometer and the source of EMF, and the reading of the galvanometer noted. The unknown resistance is then replaced with a calibrated variable resistance, and this is adjusted until the current is back to its original value. The reading of the rheostat is thus the resistance in ohms of the unknown.
Physics laboratories still have stocks of rheostats: heavy cylinders of ceramic on which a helix of wire is wrapped. A slider allows the resistance to be varied, usually to limit the current in a circuit. A typical example is shown in Fig. 2. When this rheostat was given to me, it was incredibly dirty, and I cleaned it by running it through the dish-washer! [editor’s note: not usually recommended] Rheostats of this type can be used in two ways. When connections are made to one end of the coil and to the slider, it serves as a variable resistor. On the other hand, the two ends can be connected across, say, the output of a power supply so that there is a constant EMF across the rheostat. The output can then be taken between the lower (grounded) end of the coil, and the slider. Adjusting the slider allows the user to choose any EMF between zero and the maximum, always keeping in mind that the resistance of the load must be considerably larger than the resistance of the rheostat to keep from changing the output EMF of the power supply.
Figure 3, which is in the apparatus collection at Vanderbilt University in Nashville, Tennessee, is a more refined version of the tubular rheostat. The small ivory (?) label inset into the mahogany base reads “Elliott Brothers/449 Strand/London.” The overall resistance is 4 Ohms. As the crank on the right hand side is turned, the small wheel that slides sideways on the rod across the front rolls along in the indentations in the wire. One connection is made to the rod and the other to one end of the wire.1
Somewhere in the early years of the twentieth century, probably about the time that amateurs started to build radio receivers in the 1920s, this form of rheostat acquired a new name and was referred to as a “pot”, short for “potentiometer.” This is a confusing usage, as the potentiometer is actually an instrument for measuring EMFs.2 The “pot” on the other hand, is actually a voltage divider, and is a very common element in modern electronics. Figure 4 shows a “pot” that is used to handle heavy currents and dissipate a good deal of power; this one is rated in the 25 Watt range. The ceramic base and the cement holding the turns of the coil in place enable it to handle these conditions. Smaller pots were commonly used as volume controls in vacuum-tube radios.
The instrument in Fig. 5 is a ten-turn Helipot™, made by the Heliostat Division of Beckman Instruments. In the mid-1930s, Arnold O. Beckman (1900-2004), a young assistant professor of chemistry at the California Institute of Technology, was called upon to develop what came to called the pH Meter for an industrial client.3
Part of the design was a potentiometer with a long length of wire coiled into ten turns with a sliding contact that moved around the wire as the knob was turned. Figure 5 shows a complete instrument and one with half removed to show the helical winding of the coil. The design was used in the radar sets developed at the Radiation Laboratory at the Massachusetts Institute of Technology during the latter years of the Second World War, and Beckman improved the design so that the sliding contact did not jump when the coil was subjected to high g-forces. This design continues to be made to this day in an essentially unmodified form.
- Robert T. Lagemann, “The Garland Collection of Classical Physics Apparatus at Vanderbilt University”, Folio Publishers, Nashville, Tennessee, 1983, pp 106-107. ↩
- Thomas B. Greenslade, Jr., “The Potentiometer”, Phys. Teach., (2005) 43, 232-235. ↩
- Oral Interview with Arnold O. Beckman, “Fifty Years of Beckman Instruments”, Engineering and Science, May 1986, pp 20-26. May be viewed online at: calteches.library.caltech.edu/599/2/Fifty.pdf ↩