Portions excerpted from The Sexual Response by Erick Janssen, Nicole Prause, & James H. Geer
Originally described by Fisher and colleagues (1965), the first circumferential measure, the mercury-in-rubber strain gauge, was adapted from a similar transducer used by Shapiro and Cohen (1965). The device consists of a hollow rubber tube filled with mercury and sealed at the ends with platinum electrodes. The gauge is placed mid-shaft on the penis and can easily be fitted by the participant. The operation of the mercury-in-rubber strain gauge depends upon penile circumference changes that cause the rubber tube to stretch or shorten, thus altering the cross-sectional area of the column of mercury within the tube. The resistance of the mercury inside the tube varies directly with its cross-sectional area, which in turn is reflective of changes in the circumference of the penis. Variations of the mercury-in-rubber gauge have been described by Bancroft et al. (1966), Jovanovic (1967), and Karacan (1969). This gauge is flexed when the penis changes in circumference, producing changes in its resistance. The resistance changes are in turn coupled through a bridge circuit to a polygraph or computer. Both types of strain gauge are available from Behavioral Technology, Inc. (Salt Lake City, Utah) and the rubber strain gauge is also manufactured by Hokanson, Inc. (Bellevue, Washington).
Due to the ease of its use, penile strain gauges have remained relatively popular in laboratory use. A number of studies have shown that the mercury-in-rubber strain gauge demonstrates linear outputs, high test-retest reliability, high stability over time, and minor sensitivity to temperature (e.g., Karacan, 1969; Farkas et al., 1979; Earls and Jackson, 1981; Richards et al., 1985; Richards et al., 1990; Janssen et al., 1997). Nowadays, the mercury-in-rubber strain gauge is also available in versions filled with an indium-gallium alloy, which is considered to be even less sensitive to temperature than mercury (Richards et al., 1985).
Because differences in individual penis size cause differential circumferential changes during sexual arousal, physiological sexual responses measured with strain rubber strain gauges should be standardized within-participants by converting all data points to either percentage of full erection (PFE) or Z-scores. PFE is calculated by the equation 1 – [(max – χ)/(max – min)], where max denotes the largest circumferential value (full erection) and min denotes the smallest circumferential value (flaccid state) observed during the experimental session. The value of χ corresponds to a particular circumferential data point.
A potential concern with the use of circumferential measures is the suggestion that penile circumference may show a slight decrease at the onset of sexual arousal (McConaghy, 1974; Abel et al., 1975; Laws and Bow, 1976; see also Kuban et al., 1999). A brief decrease in circumference may represent a problem in that it may be incorrectly interpreted as a decrease in sexual response. Further, it has also been noted that strain gauges may be unreliable at the upper end of the tumescence curve (Earls et al., 1983). This may represent a limitation if the measures are to be used for determining the full range of erectile capacity.