A slide rule is an instrument for multiplying or dividing numbers by the addition or subtraction of their logarithms. On a rule the numbers 1 to 10 are marked along the scale at distances proportional to their logarithms, so that two numbers can be multiplied by just adding two lengths together. On a typical 30 cm rule numbers can be read to a precision of about 1 in 500. In the cylindrical slide rule the scale is wrapped 50 times around the cylinder in a helical fashion giving a total scale length of about 10 m, with a corresponding improvement in precision to 1 in 15,000.
There are brass marker bars that can be moved round and along the cylinder. One number is set up between the markers, and the cylinder is then moved to add the second number to it and the product is read off on the scale.