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Virtual Slide Rule
See how calculations used to be done before the days of electronic calculators. Find out about an important piece of engineering history. All pieces of the slide rule are draggable.
[ Flip Slider ]
The Slide Rule
Before electronic hand held calculators, the slide rule was widely used in Engineering, Science and Commerce for rapidly performing calculations involving multiplication and division which have to be accurate to not more than three or four decimal places.
It can also be used for such operations as involution (raising to a power) and evolution (extraction of a root) and for calculations with trigonometric functions (sine, cosine, tangent, cotangent).
A standard slide rule consists of the actual rule, the slide and the transparent cursor with a hair line. Various logarithmic scales are engraved on the rule and the slide. When the rule is "closed", the pairs of scales A & B and C&D respectively, coincide.
Example of a simple MULTIPLICATION
Use scales C & D
The index line on scale C is always put over the number to be multiplied on scale D. The answer is then read off scale D, below the multiplying, number on scale C, using the cursor line for ease and accuracy
Exercise : multiply 13.2 by 2, by 6, by16, by 9, by 72
N.B. the decimal point
When starting a calculation it is as well to make a mental note of where you expect the decimal point to appear in the answer, or if necessary, do a rough longhand reckoning to find out. For instance the Slide Rule's answer to 3.75 x 8.95 is 336. If we "round up" to 4 x 9, we know that the answer is 36. Clearly the answer then is 33.6.
Example of a simple DIVISION
Use scales C & D
This is very simply achieved by sliding the divisor on scale C over the number to be divided on scale D and reading the answer off on scale D under the left or right hand index
e.g. 8 divided by 2 = 4
Slide 2 on scale C over 8 on scale D using the cursor to help. Read off 4 on scale D under the L.H.I.
Exercise : divide 48 by 8, 392 by 14, 110 by 5, 2468 by 3.7