Inside a Bathroom Scale
 

In this figure you can see that a long lever, fixed at the left end, has weight applied to it very close to the fixed end. At the right end of the lever the weight is significantly reduced by a ratio determined by the relative difference between the length from the left end to the weight and the distance from the left end to the right end. As you can see in this diagram, the ratio is 1:12. So if a 120-pound person stands on the scale, only 10 pounds is "felt" at the right hand end of the scale. The following three figures show the arrangement of the levers in this scale:

The two main levers end at a plate that houses the scale's main spring. Here you can see why a 1:12 ratio is used in the scale - it significantly reduces the size of the spring. The spring only has to be able to support 20 or so pounds. Without the levers, the spring would have to support 240 pounds, and it would have to be much bigger to do that. It would also have to be a lot more precise.


 
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