11. Mechanical calculators remained in use until the 1970s.12. Old mechanical calculators had a fixed number of decimal digit positions but no sign. 13. In 1673, Gottfried Leibniz demonstrated a digital mechanical calculator , called the Stepped Reckoner. 14. This method was commonly used in mechanical calculators and is still used in modern computers. 15. For forty years the arithmometer was the only type of mechanical calculator available for sale. 16. Blaise Pascal invented the mechanical calculator in 1642; it was the first operational adding machine. 17. It has been on display surrounded by the mechanical calculators that it was to make obsolete. 18. It was over 1, 000 times faster than the mechanical calculators available at the time. 19. Its user interface was used throughout during the 120 years that the mechanical calculator industry lasted. 20. One of their last mechanical calculators is the HL-21, an Odhner type machine.
