This is one of Jung's early clinical experiments utilizing a new technique created by American Psychologists and is one of his earliest publications. This is the second academic paper he published in English. Originally published in English in the "Journal of Neurology" in London in 1907, this edition has updated the manuscript has been updated into American English for readability. This new edition contains an Afterword by the Translator, a philosophic index of Jung's terminology and a timeline of his life and works. Jung's experimental research into the association method and the psychogalvanic reflex, his study of mediums and his links with Swiss psychiatry had many unique alliances with the American scene, particularly because of the similar historical relationship between psychology and religion. To understand Jung, therefore, one must consider the archetypal significance of America for Jung's own process of individuation, as well as the subsequent Americanisation of Jungian ideas. The galvanometer is an instrument that measures electrical conductance, while the pneumograph measures respiratory function. The study aimed to assess physiological responses in both normal and mentally ill subjects when exposed to different stimuli and situations. The results of Jung's experiments provided insights into how physiological responses, such as changes in skin conductance (measured by the galvanometer) and respiratory patterns (measured by the pneumograph), may differ between people with and without mental illness. These differences may have helped researchers and clinicians to better understand the psychophysiological aspects of mental disorders and may have contributed to the development of diagnostic and therapeutic approaches in the fields of psychology and psychiatry. Jung was particularly interested in how these physiological responses, such as changes in skin conductance and respiration rates, might correlate with emotional states and unconscious psychological activity. This research aimed to demonstrate that mental illnesses could manifest in measurable physical reactions, providing empirical evidence for the existence of unconscious emotional complexes. The galvanometer, for instance, detected changes in the electrical resistance of the skin when subjects were presented with emotionally charged words during a word association test, reflecting the activity of hidden complexes. The study had two primary goals. First, it sought to illustrate the link between physiological responses and emotional states in both normal and pathological conditions, thereby lending scientific support to psychoanalytic theories about the unconscious. Second, Jung aimed to show that these physiological measures could be used to differentiate between mentally ill and healthy subjects, offering potential applications in clinical diagnosis.