# Resistor Cube Equivalent Resistance - Another Approach(and for other geometries)Kirt's Cogitationsâ„˘ #361

"Factoids," "Kirt's Cogitations," and "Tech Topics Smorgasbord" are all manifestations of my ranting on various subjects relevant (usually) to the overall RF Cafe theme. All may be accessed on these pages:

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Resistor Cube Equivalent Resistance

Probably the most responded-to article I have ever written is the "The Resistor Cube Equivalent Resistance Conundrum," aka Kirt's Cogitationsâ„˘ #256. Having equal resistances in all branches usually makes things easier. An alternate method of solution, posted here, was provided by RF Cafe visitor Les Carpenter.

A couple days ago, another approach to the solution was submitted by Mr. John Crabtree. As with the others, his work demonstrates an intuitive way of looking at the circuit that simplifies a solution. Whereas my method invoked knowledge of currents into and out of nodes, then dividing that current into a voltage applied across two opposing corners, John combines parallel and series resistances until a final single equivalent resistance emerges.

I like his old school pencil and paper presentation rather than a computer drawing. It reminds me of the days when teachers wrote on a transparency on an overhead projector (while students frantically tried to copy it down and comprehend what is being discussed at the same time).

This goes to show how there is usually more than one way to solve a complex problem, and no version necessarily earns the Occam's Razor title.

April 3, 2024 Update for Polyhedral Resistive Structures:

Not being satisfied with merely providing the solution for the resistor cube, Mr. Carpenter followed up with an application of the "Equivalent Resistors of Polyhedral Resistive Structures" method shown to the right. Note, again, everything hand-written. He states:

Equivalent Resistors of Polyhedral Resistive Structures

John also provided a hyperlink to a paper on the ResearchGate website entitled "Equivalent Resistors of Polyhedral Resistive Structures," in which author F. J. van Steenwijk (University of Groningen, the Netherlands), provides a method for solving any regular geometric solid shape with equal resistances along each edge, being connected at the corners.

Nodal symmetry for resistor cube (6 faces, 8 vertices, 12 edges).

Nodal symmetry for resistor regular dodecahedron (12 faces, 20 vertices, 30 edges).

Nodal symmetry for resistor regular icosahedron (20 faces, 12 vertices, 30 edges).

The paper by F. J. van Steenwijk is beautiful. Having a current of I / (H - 1) leaving the remaining terminals is a wonderful insight.

Before you post anything, I should tell the other part of the story.

This puzzle has been around for over a century. For instance it appeared in Brooks, E.E. and A. W. Poyser, Electricity and Magnetism: A Manual for Advanced Classes, Longmans, Green, 1914. I found this reference [for a Resistor Tesseract]. It was also in the 1912 edition of the same book. I hav[n't] seen any earlier references to it.

And yet there seem to be very few efforts to draw the circuit as a planar network which shows the three planar axes of symmetry through the cube.

I set out to do this, and considered what the cube would look like if it were drawn on a great circle map with the input node at the centre and the output on the other side of the sphere. That got me to Fig. 2 on the cube page in the attached file. I then straightened some of the lines which got me to Fig 3. The three planes of symmetry in the cube are now very apparent, which makes the points of equal voltage apparent. This makes the parallel resistor solution very straightforward.

Fig. 3 also makes it very straightforward to seen the current distribution, and derive a solution by summing voltages.

I suggest to you that Fig. 3 makes the problem very much easier, if not trivial.

I then wondered if others had tried to draw the network in the same way. I did find Fig. 2 in: i) "Symmetry in Science: an introduction to the general theory", Joe Rosen, Springer-Verlag, 1995, pp 110-115. ii) "A Symmetry Primer for Scientists", Joe Rosen, John Wiley, 1983, pp 124-129 iii) "Symmetry discovered : concepts and applications in nature and science", Joe Rosen, Cambridge University Press, 1975, pp 114-117. You can read all of these books at http://www.archive.org. This is as much as I have found.

I have also drawn diagrams for the same problem for the icosahedron and the dodecahedron. Again the planes of symmetry and hence the equipotential points are readily apparent.

In closing, in my diagrams the points of equal voltage lie on concentric circles. These circles are equivalent to the equipotential planes in Fig. 1 of van Steenwijk's paper. I see things better in two dimensions!

73 John KC0G