Compared to the portable electronic digital calculators that were introduced in the early 1970s, slide rules had various advantages and disadvantages.
- The spatial, manual operation of slide rules cultivates in the user an intuition for numerical relationships and scale that people who have used only digital calculators often lack. Since users must explicitly note the order of magnitude at each step in order to interpret the results, they are less likely to make extreme calculation errors; users are forced to use common sense and an understanding of the subject as they calculate. Since order of magnitude gets the greatest prominence when using a slide rule, and precision is limited only to the few digits that are normally useful, users are less likely to make errors of false precision.
- When performing a sequence of multiplications or divisions by the same number, the answer can often be determined by merely glancing at the slide rule without any manipulation. This can be especially useful when calculating percentages (e.g. for test scores) or when comparing prices (e.g. in dollars per kilogram). Multiple speed-time-distance calculations can be performed hands-free at a glance with a slide rule.
- Other useful constants such as pounds to kilograms can be easily marked on the rule and used directly in calculations.
- A slide rule does not depend on electricity or batteries.
- The principle of operation of a slide rule can be demonstrated with a pair of hand-made paper scales.
- A slide rule displays all the terms of a calculation along with the result. This eliminates uncertainty about what calculation was actually performed.
- A slide rule is physically more durable than an electronic calculator and is impervious to moisture and immersion in water.
The professor who taught me physics was from the slide-rule generation. He could do calculations like sin(4)/1.76 in his head.