Hewlett Packard (HP) launched in early 1972 the first scientific pocket calculator. It was unlike the other basic four-function pocket calculators then available in that it was the first pocket calculator with scientific functions that could replace a slide rule. The HP-35, along with nearly all later HP engineering calculators, used reverse Polish notation (RPN). A calculation like "7 plus 3" is, using RPN, performed by pressing "7", "Enter↑", "3", and "+"; instead of the algebraic infix notation: "7", "+", "3", "=").
The HP-35 and similar scientific calculators by Texas Instruments soon thereafter signalled the demise of the slide rule among science and engineering students. 100,000 HP-35 calculators were sold in the first year, and over 300,000 by the time it was discontinued in 1975. Introduction price in the US was $395, in Belgium €328 (13.231 BF).
Death of a Slide Rule