There are many plans for making theremins, and there are kits available, and there are some pro quality ones also available, complete with beautiful cabinetry...
heterodynes one fixed oscillator against one variable oscillator, both above the hearing range, with the variable oscillator using capacitance of the hand to vary the resonant freq...other antenna used hand capacitance to vary amplitude of beat freq...
...can sound like a woman singing without words...with some soundwave shaping electronics, it can be a pretty versatile instrument. Takes a skill akin to Trombone or Violin playing - good tone control and some really quick gestures to cut one note and begin the next.....
Not usually a sphere in shape...most common feature of the instrument was that it had two antennae, often a plate and a rod, no matter what shape the antennae, they were nearly always arranged so that one was sensitive at right angles to the sensitive orientation of the other...Have seen rod/rod, rod/plate, plate/plate, and loop/rod arrangements.
Shapes were usually a box, with one a horizontal cylinder...
Was commonly used in 50's and 60's sci-fi movies and shows....
