Slide Rules Use Trigonometry to Simplify Calculations

A slide rule is an analog device that lets you multiply and divide by sliding logarithmic scales. The trick is to add logarithms and then read the antilog, which is why 2×3 is the same as 2×3000 in the same way. The challenge is that adding or subtracting is not straightforward, and you need a sense of scale.

Turning additions into trigonometry

Bob the Science Guy shows how to turn a difficult addition into a trigonometric problem that a slide rule can handle more easily. By using sine or cosine relationships, the addition becomes a multiplication of logarithms on the slide rule, bypassing the awkward direct addition. This trick makes the slide rule more practical for everyday calculations.

The article encourages readers to try the technique with a modern browser‑based slide rule that shows both sides in full screen. By following Bob’s videos, anyone can apply this trigonometric shortcut and bring an old slide rule back into everyday use.