The Rikitake-type disk dynamo, consisting of two identical single Faraday-disk dynamos of the Bullard type coupled together, has been analyzed intensively for more than thirty years by: (a) mathematicians interested in its ability to behave chaotically, and (b) geophysicists needing low-dimensional electromechanical analogues of the geodynamo in their efforts to elucidate the complex time series of geomagnetic polarity reversals over geological time. Implicit in the theory is what now appears to be an unwarranted assumption, namely that mechanical friction is unimportant both qualitatively and quantitatively as a dissipative agency in comparison with ohmic effects, and can therefore safely be neglected. The assumption is inconsistent with the findings of the present study, which shows that mechanical friction can render the Rikitake dynamo ''structurally unstable'' and consequently incapable of producing chaotic oscillations. Research on low-dimensional analogues of the geodynamo should concentrate attention on structurally stable systems, with properties less dependent on the starting conditions. ¿ American Geophysical Union 1995