Arithmometer. Invented in 1820 and manufactured from the 1850s to 1915, these were the first widely used mechanical calculators. This is a Payen Type P3 arithmometer, serial number 3149, manufactured in 1897 by the company of instrument maker Louis Payen of Paris, France. It has a calculation capacity of 8x9x16, referring to the eight columns of 10 at lower right, the nine digits above that, and the 16 digits across top. The switch at centre left alternates between addition and multiplication, and subtraction and division. Photographed at the National Physical Laboratory, Teddington, UK.