Newton'S Third Law - Static Dynamometers
To demonstrate action-reaction in the static case or, alternatively, to show that the sum of two forces on a point in equilibrium is zero.
The dynamometer springs have been arranged so that the upper meter measures the force pulling down on it, while the lower meter measures the force pulling up on it, including its own weight. Pulling down on the hook below the lower meter results in a pair of equal and opposite forces acting on the coupling washer.
Bill Konrad, Demonstration to Illustrate Newton's Third Law, Idea Bank Collation, Idea No. 120. Mario Iona, Misunderstanding the third law, TPT 18, 219, (1980).