Here’s a two-fer image for my visual dictionary of early domestic items: it shows a set of steelyards (aka stillards or Roman balance – the long object in the upper left of the image below) and a scale (image right):
Steelyards and scales both weigh objects. So why have two machines that do essentially the same thing? The reason seems to be one of scale. That is, size.
Scales (equal-arm balances) are useful for weighing smaller items, such as coins or spices. However, if you want to weigh anything larger than a few pounds you’d need to build a bigger and bigger scale. Steelyards (unequal-arm balances) are often used to weigh larger or heavier items without needing to be equally big.
To illustrate, it’s a more efficient use of space and materials to weigh a person with a steelyard balance such as this:
Than to use a person-sized scale, like this:
So how do steelyards work? You suspend whatever you’re weighing from a hook or loop on the short side of the pivot. Then slide the counterweight away from the hanging hook on the longer side, which is marked out in pounds, until the indicator, an arrow fixed at the same spot as the hanging hook, points directly upwards (perpendicular to the floor). It looks kind of like the image below (but only kind of).