Setup: Two cords attached to the ends of a spring scale. The other ends of the cords pass over two pulleys and are connected to two 1 kg weights so that the scale is suspended horizontally and the weights hang vertically. What does the scale read?

Observations:  

• Although the net force on the scale is zero - it is in static equilibrium - the scale does not read zero because a scale is designed to measure the tension in the attached cords and not the net force on the scale.
• The scale does not read twice the force of the hanging weights just because it is being pulled in two opposite directions by the same force. If this were true then a vertically hanging scale should also read twice the weight hanging from its bottom since the tension in the top cord has to be the same - again it's in static equilibrium.
• Internally the spring in the scale stretches until the tension in the spring is equal to that of the attached cords. The physical stretching of the spring caused the pointer on the scale to rotate. Since the force exerted by the spring is directly proportional to its stretched length (Hooke's Law), the scale can be calibrated to measure the spring force and thus the tension in the cords attached to it.