I have come across several answers on the internet, which address the question "Does a weighing scale measure mass or weight?" but I assure you that this is not one such question.
My doubt is this; while solving problems involving free body diagrams of weighing machines, when asked to find the reading of the scale, I'm a bit confused; exactly which force does the scale measure? The $mg$ downwards, due to the weight of the body on the scale alone or the normal force? I need this concept to solve problems such as the one that the following picture describes, in which I am required to find the reading of the weighing scale marked (1).
Please help! Much thanks in advance :) Regards.