Slide rules are analog computing devices that use logarithmic scales to perform multiplications, divisions, roots, and other calculations through alignment of the scales. The most common slide rule, the Pickett 120, contains multiple inner and outer scales labeled with letters to calculate operations like multiplication, reciprocals, square roots, trigonometric functions, and logarithms through simple alignment of the appropriate scales. Proper use of the slide rule requires understanding of the mathematical concepts like exponents and logarithms that it is based on.

1. Old School Slide rule for fun and profit. Or An elegant computer from a more civilized age.

2. Slide rules are based on logarithms: Two logarithmic scales next to each other will align at the product. Therefore, align the value of the first operand with the “one” value, the position on the second scale of the second operand aligns with the product of the two numbers. In 1614, John Napier discovered the logarithm which made it possible to perform multiplications and divisions by addition and subtraction. Ie: a*b = 10^(log(a)+log(b)) and a/b = 10^(log(a)-log(b)) First Principles

3. Aligning Scales Two rulers aligned will perform addition: Line up the 0 position with the first value and then find the value across from the second. Two logarithmic scales will perform multiplication. `

4. My Slide Rule 2 x 2 = 4

5. You can also use non-actual values with simple magnitudes of 10 to estimate values out of the immediate range to the rule. Here, we have removed the 10 from the 16.6 and treated the 4 mark as 10, that means the product must take back the power of 10 we cut from the C scale. Actual slide rule starts at 0, so you could point to actual 16.6 Advanced

6. Pickett == Company that made slide rules Pickett Scales == Most common labels on the various scales. The C and D lines are as above, but the Pickett 120 has three other inner scales and three other outer scales. Pickett

7. A 1960 Pickett manual said: "When people have difficulty in learning to use a slide rule, usually it is not because the instrument is difficult to use. The reason is likely to be that they don't understand the mathematics on which the instrument is based, or the formulas they are trying to evaluate. Some slide rule manuals contain relatively exhaustive explanations of the theory underlying the operations. In this manual it is assumed that the theory of exponents, of logarithms, of trigonometry, and of the slide rule is known to the reader, or will be recalled or studied by reference to formal textbooks on these subjects." From the Pickett Manual

8. C + D = Multiplication CI + D = Reciprocals CI = 2, D = 5 == .5 since .2 is the reciprocal of .5 A + B = Square Roots Align the base on A, and look across to D to find The square root. S + D = Sine and Cosine. This scale runs in reverse and aligns angles on D. T + D = Tangent (See above) L + Any= Log base 10 of value – this is simple a linear scale like a ruler! Pickett Scales

9. C + D = Multiplication CI + D = Reciprocals CI = 2, D = 5 == .5 since .2 is the reciprocal of .5 A + B = Square Roots Align the base on A, and look across to D to find The square root. S + D = Sine and Cosine. This scale runs in reverse and aligns angles on D. T + D = Tangent (See above) L + Any= Log base 10 of value – this is simple a linear scale like a ruler! Pickett Scales