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This paper presents for the first time the development of a tunable (i.e., variable) ultracapacitor. One set of metal plates, coated with a thin layer of carbon nanotubes, constitutes a stationary electrode. The stationary electrode is permanently immersed in an electrolytic solution. A second electrode, similar in structure to the first, is movable in and out of the electrolytic solution. The capacitance of such an ultracapacitor will be proportional to the immersed surface area of the movable electrode. In the prototype that was assembled by the authors, the capacitance varies from 0 to 4 F. The tunable ultracapacitor will be useful in a number of applications, including ultrawideband radio transceivers and electromechanical transducers with high sensitivity.