HP-35 Scientific Calculator, 1973

In 1971, William Hewlett challenged his engineers to miniaturize the company's 9100A Desktop Calculator--a forty-pound machine--into a device small enough to fit into his shirt pocket. The result--the HP-35--was the world's first handheld scientific calculator. It was expensive, but its powerful processing capabilities made it a rapid success, causing the swift abandonment of the slide rule.