Everyone has a scale in their heart, a sense of right and wrong.
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As a noun for the weighing instrument it is always chèng: 电子秤 (digital scale), 杆秤 (steelyard). A rare reading chēng exists as a variant of 称 'to weigh', but in modern use that verb is written 称.
culture
The traditional Chinese steelyard 杆秤 gave rise to the saying 心里有杆秤: everyone carries their own internal scale of judgement.
禾 on the left is a stalk of grain with a drooping head. It marks 秤 as a tool tied to farming, the balance first used for weighing grain at market. Same radical: 称 to weigh, 租 to rent, 稻 rice.
平 on the right supplies the sound, píng drifting to chèng, and also contributes meaning: a balance is the instrument that brings two pans to a 平 level. Few phono-semantic compounds fit this cleanly on both counts.
