The Mosso plethysmograph was designed by Angelo Mosso in 1874. He used it to detect and record blood pressure changes. This specimen was made by Charles Verdin, France, 1870-1890

A person would put his arm in a glass tube filled with water. As his blood pressure increased or decreased, changes in volume in the tube resulted in water displacement, the amount of which was transferred to a stylus and recorded on a revolving drum.

This "graphic method" was a movement that allowed scientists to better record and compare otherwise unobservable small muscular movements and pressure and volume changes of the body.

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All images courtesy of the Bakken Museum