A Penile Plethysmograph is used to measure bloodflow to the penis. Bloodflow to the penis of course indicates sexual arousal.
The two main types of penile plethysmograph:
- Volumetric Air Chamber (measures displaced air in the chamber to see how much more space the penis is taking up)
- Circumferential Transducer (a ring or strain gauge is placed around the shaft and measures changes in the circumference of the penis)
Plethysmographs have been used widely in research. Usually participants will be strapped up to a machine and shown various images or video, then the plethysmograph can measure the degree of arousal that each image provokes.
One of my favourite ever studies using this method found that homophobic men show greater arousal to depictions of gay sex than less/non-homophobic men.