Freund (1963) was the first to develop an air volumetric plethysmograph. "Less widely used variants of this technique have been described by Fisher et al. (1965), who used water instead of air, and McConaghy (1976). The plethysmograph is positioned on the participant by the experimenter, who places a sponge-rubber ring and plastic ring with an inflatable cuff over the penis. A glass cylinder with a funnel at the top is fitted over the other components, and the cuff is inflated with air. Changes in the size of the penis result in displacement of air, which can be detected by a pressure transducer."From Janssen, E., Prause, N., & Geer, J. (2007). The sexual response. In J. T. Cacioppo, L. G. Tassinary, & G. G. Berntson (Eds.), Handbook of Psychophysiology, 3rd ed. New York: Cambridge University Press.