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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 41th: 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.


  1. ^ A Discourse of Coin and Coinage
  2. ^ Letter of Isaac Newton dated September 19, 1698, to John Locke, concerning the weight and fineness of various coins.