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