FAQ
TL;DR: Yes—people still use slide rules for quick, 3‑sig‑fig work. One engineer recalls “two weeks (8 hours/day) of calculator training,” and warns: “The use of calculators…has ruined man’s ability to manipulate numbers.” [Elektroda, David Adams, post #21662988]
Why it matters: If you tinker, study, or work where batteries die or devices are banned, a slide rule stays fast, durable, and EMP-proof.
Quick Facts
- Typical accuracy: about three significant digits, fine for quick engineering estimates. [Elektroda, Richard Comerford, post #21662989]
- Core functions: multiplication, division, roots, logs, and trig; not for addition/subtraction. [Elektroda, madhurika Shrivastava, post #21663002]
- Obsolescence timeline: handheld scientific calculators displaced slide rules around 1974. [Elektroda, madhurika Shrivastava, post #21663002]
- Virtual tools exist (e.g., N909-ES simulator) for practice without hardware. [Elektroda, Joe Wolin, post #21662985]
- Phone apps and web calculators (eCalc; RPN support) offer modern alternatives. [Elektroda, Joe Wolin, post #21662995]
Do people still use slide rules instead of calculators or computers?
Yes, mainly for quick estimates, education, or as a no‑power backup. Several posters still own and occasionally use slide rules for fast multiplicative tasks and voltage‑divider ratios. One keeps a slide rule “In Case of Emergency, Break Glass” above his workstation, underscoring its backup role. [Elektroda, Richard Comerford, post #21662989]
How accurate is a slide rule in practice?
Expect about three significant digits. That level suits resistor ratios, scaling laws, and sanity checks. One engineer notes using a slide rule for fast decimal multiplication when only three‑digit accuracy was needed. For tighter tolerances or statistics, use a scientific calculator or software. [Elektroda, Richard Comerford, post #21662989]
What calculations are slide rules good at—and not good at?
They excel at multiplication, division, roots, logarithms, and trigonometry. They do not perform addition or subtraction directly. Treat them as an analog computer for proportional reasoning and order‑of‑magnitude checks. For sums and differences, switch to pencil‑and‑paper or a calculator. [Elektroda, madhurika Shrivastava, post #21663002]
How do I quickly compute a resistor divider with a slide rule?
- Set the C scale 1.0 under the top resistor value on the D scale.
- Move the cursor to the bottom resistor value on C.
- Read the ratio R2/(R1+R2) or R1/R2 as needed, then apply to input voltage for Vout.
“Great for resistor ratios as a voltage divider.” [Elektroda, Floy Viola, post #21662982]
Is there a free way to learn without buying a slide rule?
Yes. Use the online N909‑ES virtual slide rule to practice movements, cursor alignment, and reading scales. It mirrors classic linear rules and includes guides. Start with multiplication/division, then try logs and square roots. [Elektroda, Joe Wolin, post #21662985]
What does RPN mean, and why do engineers mention it with calculators?
RPN stands for Reverse Polish Notation, a stack‑based entry method used by classic HP calculators and favored by some engineers for speed. It also appears in the Forth language, which uses stack operations for calculations. [Elektroda, Steve Lawson, post #21662998]
Why is it called “Polish” notation?
It references logician Jan Łukasiewicz, who introduced Polish notation; RPN is the reversed form. A poster quips it’s named because it goes “against that of Poland,” linking to background on Polish notation. [Elektroda, Joe Wolin, post #21662999]
When did slide rules fall out of mainstream engineering use?
Around 1974, affordable handheld scientific calculators displaced them in classrooms and labs. By the mid‑1970s, most suppliers exited the market as engineers adopted TI and HP calculators. [Elektroda, madhurika Shrivastava, post #21663002]
Are there smartphone apps or modern tools that mimic slide rules?
Yes. Users report slide‑rule apps on smartphones and recommend web calculators like eCalc, which supports algebraic and RPN modes and complex numbers. These help practice workflows and handle advanced math when precision is required. [Elektroda, Joe Wolin, post #21662995]
How steep is the learning curve compared to a calculator?
It’s approachable. One member trained two weeks at 8 hours per day on calculators, yet still values mental math and tables. A slide rule builds number sense through estimation and scale reading. “The use of calculators…has ruined man’s ability to manipulate numbers.” [Elektroda, David Adams, post #21662988]
What’s an edge case where a slide rule beats digital tools?
During power loss, device bans, or EMP events, a slide rule still works. One commenter joked these will be “worth their weight in gold” after a nuclear or EMP event, highlighting resilience as the differentiator. [Elektroda, Joe Wolin, post #21662985]
Which brands or models do posters remember using?
Mentions include HP (RPN heritage) and TI calculators arriving during junior years, which rapidly replaced slide rules. Some keep Ricoh slide rules and display them as memorabilia, yet still use them for quick decimal math. [Elektroda, Richard Comerford, post #21662989]
What are common slide‑rule scales, and what does the P scale do?
Common scales include C/D for multiplication/division, and trig/log scales for angles and logs. Posters reference comprehensive scale guides that explain specialized scales like P, which assist with certain trigonometric relationships and powers on some models. “See the slide‑rule scales guide.” [“Slide Rule Scales – HP Museum”]
Can universities still require slide‑rule use?
Some instructors assign a slide‑rule exercise to build estimation and logarithmic intuition. One poster notes a professor incorporating a slide‑rule project in an online engineering course, then returning to modern tools. [Elektroda, Gregg Williams, post #21663001]
Is mental math still useful if I rely on calculators?
Yes. An experienced engineer reports often rounding and calculating in his head within 10% of a calculator result, using a calculator only for logs. This keeps intuition sharp and reduces over‑reliance on devices. [Elektroda, David Adams, post #21662988]
I learned on tables and semi‑log paper—does that still matter today?
Yes. Skills like reading tables, estimating square roots, and hand‑drawing Bode plots build insight into systems. Several posters reminisce about log tables and manual plotting, which complement modern tools like Excel. [Elektroda, David Adams, post #21662988]
