A Jacobus is an English gold coin of the reign of James I, worth 25 shillings.[1] The name of the coin comes from the Latin inscription surrounding the King's head on the obverse of the coin, IACOBUS D G MAG BRIT FRA ET HI REX ("James, by the grace of God, of Britain, France, and Ireland King").
Isaac Newton refers to the coin in a letter to John Locke:
The Jacobus piece coin'd for 20 shillings is the : part of a pound Troy, and a Carolus 20s piece is of the same weight. But a broad Jacobus (as I find by weighing some of them) is the 38th part of a pound Troy.[2]
These correspond to masses of 9.10 and 9.82 grams respectively, making the broad Jacobus slightly heavier.