The slide rule is a mechanical calculation aid.  The device makes use of logarithims to provide an easy method for multiplication, division, roots, powers, and other mathematical operations common to science and engineering.  The slide rule, like the pocket protector, became a cultural icon often used to caricature "nerds."  The device was driven into obsolescence and extinction with the advent of cheap, electronic calculators (now, themselves obsolete with the advent of smart phones and other, more versatile personal electronic devices).