Understanding the CI Scale
The CI scale is one of the defining features of the Post Versalog 1460. The letters CI mean C Inverted. The scale is the C scale printed in reverse order, allowing reciprocal values to be read directly.
Using the CI scale reduces unnecessary slide movements and allows many calculations to be completed more efficiently, particularly division and chained calculations.
How the CI Scale Works
A normal C scale increases from left to right. The CI scale increases in the opposite direction because it represents the reciprocal relationship of numbers.
The reciprocal of a number is found by dividing 1 by that number.
For example:
The reciprocal of 4 is 0.25 because 1 ÷ 4 = 0.25.
Finding Reciprocals Using CI
To find the reciprocal of a number using the CI scale:
- Locate the number on the CI scale.
- Place the cursor hairline over the value.
- Read the corresponding reciprocal value from the D scale.
- Confirm the decimal placement by estimation.
Division Using the CI Scale
The CI scale provides an alternative method for division. Instead of moving the slide to create a divisor relationship, the reciprocal is used directly.
To calculate a division problem using CI:
- Place the divisor on the CI scale under the dividend on the D scale.
- Move the cursor to the required value.
- Read the result on the D scale.
- Check the expected magnitude.
Example: Using CI for Division
To calculate 100 ÷ 4:
- Locate 4 on the CI scale.
- Align it with 100 on the D scale.
- Move the cursor to the reference position.
- Read the result from the D scale.
The slide rule provides the numerical relationship. The user determines whether the answer should be approximately 25, 2.5 or 250 by using estimation before accepting the result.
Continuous Calculations Using CI
The CI scale is especially useful for calculations involving multiple operations because it reduces the need to repeatedly reposition the slide.
Experienced users can combine multiplication and division operations efficiently by selecting the correct scale and maintaining the cursor position.
CI Scale Technique
The Post Versalog approach encourages understanding the relationship between scales rather than memorising isolated movements.
- Estimate the expected answer first.
- Select the correct scale.
- Align values carefully.
- Use the cursor to transfer results.
- Verify the final magnitude.
Common Mistakes
- Reading the CI scale as if it were a normal C scale.
- Forgetting that the scale direction is reversed.
- Ignoring decimal placement.
- Using CI without understanding the reciprocal relationship.
Recommended Diagrams
- C scale compared with CI scale direction.
- Reciprocal calculation example.
- Division calculation using CI and D scales.
- Continuous calculation workflow.
Continue Learning
Continue to Versalog 1460 Division.