Here are some Edo-period hanging marketplace weight scales: http://www4.airnet.ne.jp/sakura/hakari/hakari_fr07.html
They are ingenious and perhaps more complicated than you might think. I have seen people use them in recent times. Naturally there were ways to hide weights in these things to cheat customers. My point is, if you handed one of these over to a modern professor of physics, not only would she understand how it works, she would quickly find it is loaded. The last sentence of this essay says that even though there were strict weights and measures laws during the Edo period, the reliability of the scales was poor. - Jed
