# How do you use a slide rule?

#### pepeketua

##### New member
I have a slide rule that my dad used in high school and have no idea how to work it. I know I'll probably never need to use it but I'm just curious - how do you use a slide rule?

#### Harry_the_cat

##### Elite Member
Check out some of the numerous you-tube videos available.

#### R.M.

##### Junior Member
Google search: "Derek's Virtual Slide Rule Gallery". On that page is a link labeled "Slide rule calculations by example" that walks you through numerous calculation examples.

#### Jomo

##### Elite Member
I was lucky and never had to use a slide rule. Calculators had just become available when I was in elementary school.

I heard that Physicists/ Physics students had a hard time doing their calculations without the benefit of a calculator. That really must have been brutal!

#### HenryTorres

##### New member
I was lucky and never had to use a slide rule. Calculators had just become available when I was in elementary school.

I heard that Physicists/ Physics students had a hard time doing their calculations without the benefit of a calculator. That really must have been brutal!
I heard about slide rule but never saw in my life, thanks God because of calculators i was lucky too.

#### Subhotosh Khan

##### Super Moderator
Staff member
I was lucky and never had to use a slide rule. Calculators had just become available when I was in elementary school.

I heard that Physicists/ Physics students had a hard time doing their calculations without the benefit of a calculator. That really must have been brutal!
I was lucky and HAD to use it. Like using analog clocks - helps your sense of ESTIMATIONS.

#### Cubist

##### Full Member
One of my relatives collects unusual slide rules, ones that are tailored for a specific use. Many of them were used in alcohol production (@jonah2.0 might be interested in them!). He also has some that are used to estimate the weight of cattle (given a girth and length measurement). These are the two that stick in my mind, but I'm sure he has other types too.

#### HallsofIvy

##### Elite Member
I remember when my math dept. had a huge slide rule hung on the wall of the main office.

I not only had (and used) a slide rule but had (and used) an abacus!
(God, I am ancient!)

(I could calculate square roots with the abacus.)

#### JeffM

##### Elite Member
I was lucky and never had to use a slide rule. Calculators had just become available when I was in elementary school.

I heard that Physicists/ Physics students had a hard time doing their calculations without the benefit of a calculator. That really must have been brutal!
Actually, computation was easy with a slide rule so long as you used scientific notation and were satisfied with answers to three significant figures.

#### HallsofIvy

##### Elite Member
I have a slide rule that my dad used in high school and have no idea how to work it. I know I'll probably never need to use it but I'm just curious - how do you use a slide rule?
The use of slide rules was typically taught in the same course where logarithms were taught- logarithms being also a way to do computations of large numbers. To calculate 35400*726000 you would look up the logarithms of 35400 and 726000 in a table of logarithms, add the two logarithms then look up the number that gives that sum as its logarithm: log(35400* 726000)= log(35400)+ log(726000).

Since the advent of calculators logarithms are no longer taught except as the inverse function to exponentials.

The basic ideas are
1) log(ab)= log(a)+ log(b)

2) If you have equally spaced marks on one stick, marked 0, 1, 2, ..., equally spaced marks on another stick, then moving the "0" mark on one stick to the "2" mark then "3" mark on the first stick will be at the "5" mark showing that "2+ 3= 5".

On a slide rule 1, 2, 3, etc are not marked at equal distance but at distance given by their logarithms.
At the far left, we do not have "0" but "1" because log(1)= 0. And the distance from 1 to 2 is proportional to log(2), the distance from 1 to 3 is proportional to log(3), etc.

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#### Subhotosh Khan

##### Super Moderator
Staff member
Smithsonian wants the book returned - now!!!

#### Dr.Peterson

##### Elite Member
You must be young. I have my 18th edition sitting on a shelf beside me (though not used lately except to look up a formula).

And, yes, I carried a slide rule around in college. One value of it was as a reminder that exact values aren't as important as we think they are.

#### skeeter

##### Elite Member
You must be young. I have my 18th edition sitting on a shelf beside me (though not used lately except to look up a formula).

And, yes, I carried a slide rule around in college. One value of it was as a reminder that exact values aren't as important as we think they are.
Copied the photo from a book site ... I just remember my high school Trig teacher ordering us one for \$5 in the late 60's. Mom probably got rid of it (along with all my old Marvel comic books) after I left the nest.

#### jonah2.0

##### Full Member
Beer induced recall follows.
"The young engineer asking for computer time to solve a problem that could have been done by a few swipes on the slide rule became a humorous cliché. Many computer centers had a framed slide rule hung on a wall with the note "In case of emergency, break glass."
Mathematical Tools 2012 Franklin Coyle

I stumlbled on the movie Apollo 13 about two years ago during the moon landing anniversary while channel surfing and saw a scene which caught my attention. It was the alleged use of the slide rule at ground control during a critical period of the movie. I couldn't make up my mind at the time if it's either an exaggeration or a reality that ground control personnel would rely on a slide rule given that time was a premium during the film's crisis and given NASA's resources and access to modern technology at the time. I was about to send Denis a pm concerning the use of slide rules with this clip of Apollo 13 (at 0:38) two years ago but was shocked into numbness by Mark's rather sad news.

All this talk of slide rules made me recall another clever mathematical tool from that book by Coyle: The Planimeter

#### HallsofIvy

##### Elite Member
Actually, the idea of having to "ask for time" on a computer seems humorous and old-fashioned now. How many of us remember turning in a deck of punched cards to the computer jockeys?

#### Harry_the_cat

##### Elite Member
Put me off doing any more programming courses at uni. We used pencil marked cards in the late 70s, had to submit them to the computer jockeys, took about a week to get them back. If you had one mismarked entry you had to start the process all over again. So frustrating. Stuck to studying mathematics.

#### HallsofIvy

##### Elite Member
Put me off doing any more programming courses at uni. We used pencil marked cards in the late 70s, had to submit them to the computer jockeys, took about a week to get them back. If you had one mismarked entry you had to start the process all over again. So frustrating. Stuck to studying mathematics.
Oh, dear. In the '60s we had a room with about 6 punch card machines and we punched our own cards.

There was an article in Time Magazine listing all colleges in the United States that started with Cal Tech that had, if I remember correctly, 12 computers, then down through Harvard, Princeton with progressively fewer computers, to a long list of primarily state universities that had a single computer. There was a footnote that said "This list does not include the Massachusetts Institute of Technology which has 24."!