Slide rules for children

Quick answer

Slide rules were special tools used by scientists and engineers before calculators to help them multiply and divide numbers quickly and easily.

Learning outcome

By the end of this page, you will understand how slide rules work and why they were useful for doing maths before calculators.

• You can explain what a slide rule is and how it helps with maths.
• You can use a slide rule to multiply two numbers.
• You can tell the difference between a slide rule and a modern calculator.
• You can imagine how people did maths before computers and calculators.

Why this matters

Slide rules were a clever way for scientists and engineers to do hard maths without computers. They helped build bridges, planes, and even rockets. Learning about slide rules shows how people have always found new ways to solve problems. Today, you can still see slide rules in museums or old photos. Knowing about them helps you understand how maths has changed over time.

How it works

Slide rules work like a ruler, but with special markings that help you multiply and divide. Here’s how they work:

1. Slide rules have two long rulers that slide past each other.
2. The markings on the rulers are not regular numbers. They show logarithms, which are special numbers that make multiplication easier.
3. To multiply, you line up the numbers and read the answer from the markings.
4. Slide rules can also do division, square roots, and more!

Examples you can see

Let’s imagine you have a slide rule and want to multiply 3 by 4. Here’s how you would do it:

1. Find the number 1 on the bottom ruler.
2. Move the top ruler so that 1 lines up with 3 on the bottom ruler.
3. Look at where 4 is on the top ruler. It lines up with 12 on the bottom ruler.
4. So, 3 × 4 = 12!

Here is a simple table showing how slide rules work with small numbers:

Slide rules were used in many jobs. For example, engineers used them to design buildings and bridges. Pilots used them to calculate flight paths. Even astronauts used slide rules for space missions!

Try it as a mini adventure

Adventure: The Space Mission

You are an astronaut on a space mission to the moon. You need to calculate how much fuel you need for the journey. You have a slide rule in your toolkit. You know you need to multiply 5 by 6 to find the answer. Can you use your slide rule to solve this?

1. Find the number 1 on the bottom ruler.
2. Move the top ruler so that 1 lines up with 5 on the bottom ruler.
3. Look at where 6 is on the top ruler. What number does it line up with?
4. Write down your answer.

Now try it with 7 × 8:

1. Find the number 1 on the bottom ruler.
2. Move the top ruler so that 1 lines up with 7 on the bottom ruler.
3. Look at where 8 is on the top ruler. What number does it line up with?
4. Write down your answer.

Common mistakes

• Mistake: Thinking the markings on a slide rule are regular numbers like a ruler.
  Why it happens: People confuse the markings with regular numbers.
  Fix: Remember that slide rules use logarithms, which are special numbers that help with multiplication.
• Mistake: Not lining up the rulers properly.
  Why it happens: It can be hard to see where to line up the numbers.
  Fix: Take your time and make sure the lines match up exactly.
• Mistake: Forgetting to check the decimal point.
  Why it happens: Slide rules don’t show decimal points, so it’s easy to make a mistake.
  Fix: Estimate the answer first to check if your result is reasonable.

Parent and carer guide

Helping your child understand slide rules is a fun way to explore history and maths. Here’s how you can support them:

• Ask your child to explain what a slide rule is and how it works. This helps them remember.
• Use simple examples like 2 × 3 to show how slide rules work.
• Encourage them to try the mini adventure with space missions.
• Don’t worry if they make mistakes – they’re learning!
• Share stories about famous scientists or engineers who used slide rules.
• Look for old slide rules in museums or online to spark curiosity.

Remember, slide rules were a big deal before calculators. They helped people do maths faster and more accurately. It’s a cool way to show how maths has changed over time!

Practice questions

1. What do engineers use slide rules for?
2. How do you multiply 4 and 5 using a slide rule?
3. Why were slide rules useful before calculators?
4. What are the markings on a slide rule called?
5. How can you check if your answer is right on a slide rule?

Answers:

1. Engineers use slide rules to do fast and accurate maths for building bridges, planes, and rockets.
2. Line up 1 on the bottom ruler with 4, then find where 5 is on the top ruler. It lines up with 20.
3. Slide rules were useful because they helped people multiply and divide quickly without needing a calculator.
4. The markings are called logarithms.
5. You can estimate the answer first to check if it’s reasonable.

FAQ

What is a slide rule?

A slide rule is a tool that helps you multiply and divide numbers quickly using logarithms.

Who used slide rules?

Scientists, engineers, pilots, and astronauts used slide rules before calculators were invented.

How does a slide rule work?

It works by sliding two rulers past each other, lining up numbers to find the answer to multiplication or division problems.

Are slide rules still used today?

Not often, but they are still used in some special situations and are popular with collectors and history fans.

Can I try one at home?

It’s hard to find real slide rules at home, but you can imagine how they work or look at pictures online.