What is a slide rule?

Answer 1

Now you are making me feel old! The slide rule was an analog computing device that was used extensively prior to the creation of the digital calculator. When I started engineering school, I had to have one. There is a central 'slide' that moves in between the two edges. By matching up numbers using the slide and the marker bar that slides across all of them, you could multiply and divide numbers. It could also provide logarithms and geometric functions. They were not real accurate, but sure beat trying to figure it out by hand! Using a slide rule only gave a number without saying where the decimal point was and you had to work out the approximate value yourself. Say the slide rule answer was 784 and your rough calculation resulted in 10, the answer would be 7.84. This had a benefit of your being able to reject a silly answer that a calculator might give due to your hitting the wrong key. The device was also known colloquially known as a "slip stick".
The primary use of the slide rule was multiplication and division. They are particularly good where a series of numbers need to be converted, for example converting a column of numbers of weights in pounds to their relative weights in kilograms. They were also used for trigonometric functions, finding square roots and cube roots, work with logarithms. Forty years after learning to use one, I found out it is possible to add and subtract on them, but believe me, it is easier to do addition and subtraction on paper.