## Table of Contents

A quick guide on how to read a micrometer screw gauge. Similar to the way a vernier caliper is read, a micrometer reading contains two parts:

- the first part is contributed by the main scale on the sleeve
- the second part is contributed by the rotating vernier scale on the thimble

## Measurement Reading Technique For Micrometer

The above image shows a typical micrometer screw gauge and how to read it. Steps:

- To obtain the first part of the measurement: Look at the image above, you will see a number 5 to the immediate left of the thimble. This means 5.0 mm. Notice that there is an extra line below the datum line, this represents an additional 0.5 mm. So the first part of the measurement is $5.0 + 0.5 = 5.5$ mm.
- To obtain the second part of the measurement: Look at the image above, the number 28 on the rotating vernier scale coincides with the datum line on the sleeve. Hence, 0.28 mm is the second part of the measurement.

You just have to add the first part and second part of the measurement to obtain the micrometer reading: $5.5 + 0.28 = 5.78$ mm.

To ensure that you understand the steps above, here’s one more example:

First part of the measurement: 2.5 mm

Second part of the measurement: 0.38 mm

Final measurement: 2.88 mm

## Compensating For Zero Error

### In A Nutshell

Use the following formula:

$$\text{Correct reading} = \text{Obtained reading} \, – \, \text{Zero error}$$

where $\text{zero error}$ can be either **negative** (the “0” marking on the thimble is **above** the datum line) or **positive** ( the “0” marking on the thimble is **below** the datum line )

### Explanation For Zero Error Formula

Now, we shall try with zero error. If you are not familiar on how to handle zero error for micrometer screw gauge, I suggest that you read up on Measurement of Length.

The reading on the bottom is the measurement obtained and the reading at the top is the zero error. Find the actual measurement. (Meaning: get rid of the zero error in the measurement or take into account the zero error)

Measurement with zero error: 1.76 mm

Zero error: + 0.01 mm (positive because the zero marking on the thimble is below the datum line)

Measurement without zero error: $1.76 \, – (+ 0.01) = 1.75$ mm

The subtraction logic is similar to the method explained in How to read a vernier caliper. You can take a look and comment below, if you encounter any difficulties.

## Worked Examples

### Example 1

What is the smallest possible reading (in mm) on the thimble scale? What is the biggest possible reading?

**Show/Hide Answer**

The smallest possible reading on the thimble scale is 0.01 mm, while the biggest possible reading is 0.49 mm.

### Example 2

i) In an experiment to measure the diameter of a uniform wire using a micrometer screw gauge, a student fails to notice that with the gauge fully closed, the reading is not zero.

ii) State and explain clearly whether the omission introduces a random error or a systematic error into the readings of the diameter.

**Show/Hide Answer**

i) The omission introduces a systematic error (zero error). This is because there is a constant error in one direction only, that is, it constantly causes the readings to be either larger or smaller than the true value.

ii) It causes the readings to be precise but not accurate.

If you still do not understand the concept, there is a **very useful** simulation of the micrometer screw gauge here.