What was that apparatus? A giant mechanical skeleton with glowing red eyes, combined with a human interrogator shouting questions through a megaphone and audio-visual recording built into the skeleton's skull.
How would this work? According to the patent's author, one Helen Shelby,
The skeleton is arranged in front of the panel, with its feet resting upon the glass top of the light chamber, the lights mounted in this chamber being adapted to flood the entire skeleton from the feet up, while a second source of light mounted upon the panel over the skull of the skeleton... is adapted for additional flooding from an opposite direction, the purpose being to produce the appearance of an apparition having a translucent outer, or astral body, and a diaphanous veiling constituting the so-called aura, the lighting being of a character adapted to flood with a ghostly light and to bring out clearly the skeleton's outlines.
As the patent's diagram has it,
As a bonus, or "[t]o add to the mystification, the bulbs, forming the eyes of the skeleton, will be caused to blink, upon the subject's replying to questions while under examination, this blinking resulting from the variations in the sound waves as governed by the microphones controlling the electric circuits".
So it's one part The Shadow (first appeared in 1930, same year as this patent), one part Batman (appeared less than a decade later, and famously argued that "Criminals are a superstitious cowardly lot"), and one part Dr. Mabuse (1921).