The Harmonic Wand is a transducer-based instrument that combines physical excitation, synthesis, and gestural control. Our objective was to design a device that affords exploratory modes of interaction with the performer's surroundings, as well as precise control over microtonal pitch content and other concomitant parameters. The instrument is comprised of a hand-held wand, containing two piezo-electric transducers affixed to a pair of metal probes. The performer uses the wand to physically excite surfaces in the environment and capture resultant signals. Input materials are then processed using a novel application of Karplus-Strong synthesis, in which these impulses are imbued with discrete resonances. We achieved gestural control over synthesis parameters using a secondary tactile interface, consisting of four force-sensitive resistors (FSR), a fader, and momentary switch. As a unique feature of our instrument, we modeled pitch organization and associated parametric controls according to theoretical principles outlined in Harry Partch's "monophonic fabric" of Just Intonation—specifically his conception of odentities, udentities, and a variable numerary nexus. This system classifies pitch content based upon intervallic structures found in both the overtone and undertone series. Our paper details the procedural challenges in designing the Harmonic Wand.