Key Points

  • William Pratt was a math teacher and inventor from the seventeenth century.
  • He is most famous for his calculating device called the Arithmetical Jewell in 1616.
  • This small calculating device did not take paper like many others at the time and was small enough to fit inside your pocket.

Who was William Pratt

William Pratt was a British mathematician, inventor, and a member of the active circle of London’s Mathematical teachers in the 17th century. He had a great influence in the field of two mathematical practices, Rabdology and Logarithms.

William Pratt did lots of research on the invention of Scottish Mathematician John Napier in the 1600s. He was able to create a fast and efficient way to calculate the logarithms of Napier. He provided a way in which the logarithms of Napier could be done automatically without the need for a pen and paper.

Quick Facts

Full Name
William Pratt
Place of Birth
Fields of Expertise
calculating device

Early Life

Almost nothing is known about William Pratt’s early life.


Member of the Active Circle of London’s Mathematical Teachers

William Pratt was a knowledgeable and skilled mathematical practitioner who played a vital role as a member of the active circle of London’s mathematical teachers. It was located close to the famous Gresham College founded by Thomas Gresham in 1597. 

William and the committee were responsible for studying the nature of mathematics and coming up with modern principles, expanding on mathematical theories, and creating new concepts. Together with a team of two, William Pratt made remarkable progress in the field of Logarithms. 

What did William Pratt Invent?

Arithmetical Jewell

William Pratt designed a calculating device called the Arithmetical Jewell in 1616. John Harper and Jeremiah Drury supported him to bring his fantastic mathematical tool into shape. On 27 March 1616, Pratt and his team received a patent (privilege) “for the sole making of a table for casting accounts.” The Arithmetical Jewell performed addition, subtraction, multiplication, and division with minimal or zero errors. 

Arithmeticall Jewell book of Pratt
The book Arithmeticall Jewell of William Pratt from 1617

William’s Arithmetical Jewell was nothing more than a rudimentary mechanical reconfiguration of the convectional reckoning technique: a portable, fancier, and gentlemanly adaptation of the century-old calculation techniques like plume (manual arithmetic) and Jetons of the abacus

Additionally, Pratt’s mechanical calculator did not need paper to inscribe; for instance, instead of doing the carry-over numbers of an addition manually, you could use a small metallic stylus to “inscribe” them on the instrument’s appropriate sectors brass. Nonetheless, the reckoning method was precisely the same as the plume and jetons in its calculations process. 

The Arithmetical Jewell comprises two ivory-faced wooden tablets, 122 mm x 65 mm x 5 mm each. This pocket calculator has 14 columns on one tablet, each with small brass parallel sectors. The other has seven columns for laying out astronomical fractions to the base 60.

Arithmeticall Jewell
The Arithmetic all Jewell of William Pratt (© National Museums Scotland)

William is a great mathematician acknowledged by many scholars who found his invention helpful. According to a later account by the English antiquary, natural philosopher, and writer John Aubrey (1626–1697): 

Dr. Pell told me that one Jeremiah Grinken [a mathematical instrument maker] frequented Mr. Gunters Lectures at Gresham College: He used an Instrument called a Mathematicall Jewell, by which he did speedily perform all Operations in Arithmeticke, without writing any figures, by little sectors of brasse [or some semi-circles] that did turn every one of them upon a Center. The Doctor has the booke… he told me, he thought his name is [William] Pratt.

Although the number of pocket calculators produced by William Pratt isn’t on the record, two copies of the actual instrument still exist. One in the Department of Medieval and Later Antiquities, British Museum, and the other in National Museums of Scotland. 

William Pratt: Marriage, Divorce, Children, and Personal Life

Net Worth

Nothing is known about William Pratt’s career net worth.


No information is available about William Pratt’s marriage.


No information is available about William Pratt’s divorce.


No information is available about William Pratt’s children.


No information is available about William Pratt’s death.

William Pratt Published Works and Books

The Arithmetical Jewell

The Arithmetical Jewell was published in 1617 in London by William’s Maiesties privilege to describe the operations of its mechanical calculator. After being published, the book was granted to the inventor William Pratt under the Great Seale of England. 

Below the main title, the book has the writings, “the use of a small Table Whereby is speedily wrought, as well all Arithmetical workes in whole numbers, as all fractional operations, without fraction or reduction.”


William Pratt – Complete Biography, History, and Inventions FAQs (Frequently Asked Questions) 

Who was William Pratt?

William Pratt was a great mathematician and a member of the active circle of London’s Mathematical teachers who helped in further studies of the logarithms of Napier.

What did William Pratt invent?

William Pratt invented an efficient pocket calculator that was named the Arithmetical Jewell. 

Why did William Pratt build the Arithmetical Jewell?

William Pratt wanted to make a difference in the old and complex arithmetical operations that depended entirely on manual calculations. For this reason, he created the Arithmetical Jewell to provide an intuitive way to perform additions, subtractions, multiplication, and division of numbers. 

When was William Pratt born?

No information is available about William Pratt’s birthday.

When did William Pratt die?

No information is available about William Pratt’s death.

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