It's derived from the way the Romans measured bronze and silver bars they used for currency 2,000 years ago.

它源于 2000 年前罗马人测量他们用于货币的青铜和银条的方式。

In general, however, something more is given upon coin of a known fineness, than upon gold and silver bars, of which the fineness cannot be ascertained but by a process of melting and assaying.

然而，一般来说，已知纯度的硬币比金条和银条具有更多的价值，金条和银条的纯度只能通过熔化和化验的过程才能确定。