Objective:

To multiply two values together using the C and D scales.

General:

• The two most basic and necessary scales to work with are the C and D scales. 
• The C and D are identical scales and are single-decade logarithmic scales.
• When multiplying two values together, we make use of one of the law of logarithms, which states that the logarithm of a product is equal to the sum of the logarithms of the factors.  Mathematically, we write this as:
  
• The left or right index of the C scale may be used.  The right index is used if one of the factors goes off scale, i.e. goes too far outside the body of the slide rule.

Method:

• Align the left index of C with the first factor on D.
• Move the cursor to the second factor on C.
• Read the result on D.

If the second factor goes off scale use the right index of C instead.

Example 1:  Calculate 15.5 x 3.2

• Align C1 with D1.55
• Move the cursor to C3.2
• Read D4.96

 

Example 2:  Calculate 2.5 x 6.8

When attempting to use the left index, 6.8 goes off scale, so we use the right index of C in this example.

• Align C10 with D2.5
• Move the cursor to C6.8
• Read D17

 

Practice Questions: Calculate the following products.

1. 2.3 x 3.1 (Ans: 7.1)
2. 4.6 x 1.8 (Ans: 8.28)
3. 56.5 x 7.2 (Ans: 407)
4. 78 x 92.5 (Ans: 7210)
5. 0.065 x 0.58 (Ans: 0.0377)

Solutions: Click here.