Otis King's Pocket Calculator is a cylindrical slide rule that was sold by Carbic Ltd. in London from the 1920s (Image 1, right). This 'model K' device was intended to assist businessmen with multiplication and division. it's style and simplicity helping to make it a popular device.
Other models of the Otis King Pocket Calculator were designed to perform specific tasks. The 'model E' would have been a help to shopkeepers, as its scale converted between money (in units of pounds, shillings and pence) and weight (in units of ounces and pounds). By writing the scales on a cylinder, all models of Otis King were small enough to carry in a pocket or purse.
Similar principles were applied to this pocket watch slide rule (Image 1, left), made by French instrument maker Henri Chatelain in the late 19th century. The two sides have different scales of logarithms, sines, squares, and cubes. Both the steel pointers and faces could be wound like a clock to achieve the desired alignment. Watches were among the most valuable items one could own in the 19th century, and this model appealed to luxury while being extremely useful and compact.