On February 2, 1921, an early form of lie-detection apparatus was documented in use by law-enforcement authorities in New York City, representing one of the first recorded instances of physiological recording devices being applied in a criminal investigation. The device used in 1921 was not the later multi-component “polygraph” popularized in the 1920s and 1930s, but rather a simpler instrument that recorded bodily responses then believed to correlate with deception. Contemporary reports describe experiments with blood-volume and pulse-recording mechanisms, as well as galvanic (skin-conductance) devices, all grounded in the 19th- and early-20th-century idea that emotional states produce measurable physical changes. Background: antecedents and scientific context The scientific and medical groundwork for using physiological signs as indicators of deception stretches back to the 19th century. Clinicians and researchers observed relationships among pulse, blood pressure and emotional arousal; inventions such as the sphygmograph and the cardiograph allowed mechanical recording of pulse and blood-volume changes. In the late 19th and early 20th centuries, investigators and some physicians proposed that these instruments could help distinguish truthful from deceptive statements because lying was thought to produce distinctive autonomic responses. By the 1910s, journalists, law-enforcement officials and some inventors were experimenting with adapting such apparatuses for investigative use. The 1921 event Documentation from February 1921 indicates that New York police employed a recording device during an inquiry—reports at the time identified the apparatus as a kind of lie detector and emphasized its novelty. Newspaper coverage and police statements described attaching sensors to a subject to record pulse and other responses while the subject answered questions; investigators then examined the tracings for irregularities. The event attracted public attention because it suggested a new credential for scientific policing, though contemporaries debated both the reliability of the method and its proper role in investigations and courts. Limitations and scholarly perspective Modern historians of science and forensic practice caution that early lie-detection experiments, including the 1921 use, were preliminary and fraught with methodological problems. Instruments of the period typically recorded a narrow set of physiological signals and lacked standardized questioning protocols, controls, or statistical validation. The assumption that deception produces consistent, distinctive patterns across individuals remains contested; later developments—most notably John A. Larson’s refinements in the early 1920s and Leonarde Keeler’s additions of multiple channels (respiration, blood pressure, galvanic skin response)—gradually produced the multi-channel polygraph apparatus known in mid-20th-century practice. Even so, the scientific community has long debated polygraph accuracy, and legal admissibility varies by jurisdiction. Significance The February 1921 use is historically significant as an early, documented example of law enforcement adopting physiological recording devices to probe truthfulness. It illustrates the intersection of emerging biomedical instrumentation and policing in the early 20th century, as well as public appetite for “scientific” solutions to criminal investigation. The episode also foreshadows sustained controversies about the reliability, ethics and legal status of lie-detection techniques that continue to this day. Sources and caution Primary sources for the 1921 incident are period newspapers and police statements; subsequent scholarly histories of forensic science and the polygraph contextualize these early uses. Because contemporaneous reporting sometimes conflated different instruments and because later mythmaking has sometimes overstated early capabilities, accounts vary on precise technical details of the device used on February 2, 1921. Where exact instrument specifications or outcomes are absent from primary records, this summary avoids asserting details not supported by surviving sources.