About The Slide Rule Calculating by slide rule is becoming a lost art. Until easily programmed, user-friendly computers and calculators were invented, slide rules were the favorite tool of anyone who needed to do difficult computations. Calculators came out in the late 1960's but were initially unaffordable and not very powerful. Computers were room-sized and difficult to program: there was no such thing as an interactive computer. All practicing scientists, engineers, etc. relied upon slide rules and their use was routinely taught as part of the middle and high school curriculum. It was not until the late 1970's that calculators came into common use in the schools and slide rule computation was no longer taught. There is much to be learned from using a slide rule. A physical understanding of the addition property of logarithms: the log of the product is the sum of the logs. Concrete understanding of logarithmic scale: equal distance means equal ratios. Scientific notation is needed to multiply numbers which are less than 1 or greater than 10. Number facilitiy: in order to use a slide rule effectively, it is necessary to have some estimate of what your answer should be. Calculus connection: the addition property of logs is closely connected with the fact that the area under the hyperbola xy=1 is ln x. The activity Experiment with the addition rulers. Learn to do multiplication and maybe division with a slide rule. Optional: make a circular "slide rule". Materials Addition rulers: any pair of 12" rulers will do. You could tape over the ends so that only the first 10" show (photo). It should be easy for kids to figure out how to use them to add. The instructions are on the trifold display. The objective of the addition rulers is to lay the foundation for multiplying with the slide rule, that is, sliding the rule to add the logarithms. You will need to find a few slide rules for children to use at the activity table. In the 1960's and 1970's it used to be very easy to go to the school supplies section of the local discount store and buy an inexpensive slide rule. Now they are not to be found even in the teacher's supply stores. Some possible sources: you may be able to borrow some from parents or grandparents of children at the school hosting the math night. People who are over 40 are more likely to own one than people who are under 40. You might be able to purchase inexpensive slide rules on http://www.ebay.com; they seem to be fairly commonly available there. If you really cannot find a straight slide rule, you can make a circular slide rule by printing Circular Slide Rule from the Printable Materials section of this module. Then cut out the two circles and place the smaller one on top of the other so that both scales show. Poke the pointed half of a snap through the centers of both circles and snap the two halves together. Make sure that the snap is very carefully centered. The instructions for the straight rule can be easily adapted to this circular rule. An optional activity would be to have the children each assemble one of the circular rules to take home. It might be handy to have a calculator available so children can check their calculations against it. Conducting the activity A few copies of the instructions for using the slide rule (at least one copy for each slide rule) should be printed and put out on the table. These can be found in Printable Materials. There are three pages: "Reading the Scale", "Multiplying with the slide rule", and "Dividing with the slide rule". Children doing the activity might want to follow the examples shown on these instructions and then try some problems on their own. The volunteer staffing this activity might want to be prepared to come up with some problems. Older volunteers may already know how to use a slide rule. If not, the volunteer in charge of this display will want to spend a little bit of time figuring out how to use a slide rule and practicing some calculations before working with the children. About the history The invention of the slide rule is usually ascribed to William Oughtred. Though he seems to have been the first to come up with and carry out the idea, a couple of his contemporaries independently invented the slide rule within a year or two of Oughtred's invention. Richard Delamain actually published in 1630 a description of his slide rule, a couple of years before Oughtred published his. Both of these were circular "slide rules". Edmund Wingate alluded to a straight rule which he had invented in a 1630 publication. Reference Material Websites: The Museum of HP Calculators The Oughtred Society "Use of the Slide Rule as a Teaching Tool" "Calculators and Calculating Devices" by Judy Lasater Book: Number Machines by Forrest M. Mims Contains a discussions at a child's level of abaci, slide rules, and calculators. The explanations are very lucid. Article: "Slide Rules Rule" by Eric Levin in Discover Magazine vol 24 no. 8 p. 66. (August 2003--how timely!) Historical References: A History of the Logarithmic Slide Rule by Florian Cajori A History of Mathematics: an Introduction by Victor Katz Written 7/15/03 by Janet A. Dixon Revised 1/10/06.