A dynamometer, illustrated by (or for) the United States Sanitary Commission in 1865.
A dynamometer measures power output. For an engine, it can measure things like torque or the ability to carry weight. For a human body, it can measure how tightly your hand can squeeze, or how much your bicep can lift.
This one came from a collection in the New York Public Library that also has things like photos of the Field Relief Corps, plans of locomotive hospital cars, portraits of wounded soldiers, and tents at the Battle of Gettysburg.