Mosso Plethysmograph
(Collections of The Bakken Museum, Minneapolis)
This contraption, which dates from around 1880, was designed to measure tiny changes in the human body, including blood pressure. The patient inserted a forearm into a glass tube that floated in an alcohol-and-water mixture. Blood flow made the arm expand, pushing some of the water out and making the tube float lower, raising the counterweight and a stylus, which marked its position on a revolving-drum kymograph (not pictured). Thus, the doctor could “read” a patient’s blood pressure.
