Detail of image from Baldwin's Calculating Machine. See larger image and resize image for complete picture.
About 1875 engineer Frank S. Baldwin of Philadelphia and Willgot Theophil Odhner, a Swedish engineer and entrepreneur working in St. Petersburg, Russia, independently invented calculators using a true variable-toothed gear. This was the first real advance in mechanical calculating technology since Gottfried Leibniz's stepped drum (1673). These calculators were called "pinwheel calculators."
The greater ease of use of this technology, its general reliability, and the compact size of the equipment incorporating it caused an explosion of sales in the calculator industry.