Lesson 3, Topic 2
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# Measurement of Length / Distance (Metre Rule, Vernier Caliper, Micrometer Screw Gauge)

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## Length:

Length is the extent of space between two points. Length is measured with instruments such as tape rule, metre rule, vernier caliper, micrometer screw gauge, etc.

Smaller length distances are measured using a vernier caliper, micrometer screw gauge.

### Metre Rule:

A meter rule is a device, which is used to measure the length of different objects. It is calibrated in millimeters and centimeters.

A meter rule of length 1m is equal to 100 centimeters (cm). On the meter rule, each cm is divided further into 10 divisions which are called millimeters (mm).

Some meter rules are calibrated in inches or feet. The reading accuracy of a metre rule is 1 mm or 0.1 cm.

### Vernier Calliper:

Vernier caliper is used for measuring small lengths (0-15cm). These calipers come in handy in measuring the diameter of circular objects. It has a reading accuracy of 0.01cm. A pair of vernier calipers consists of a main scale (m) and a vernier scale (s).

The outside jaws are used to measure the outer dimensions of an object and the inner jaws to measure internal or inner ones. To measure, gently grip the object with the straight edges of outside or inside jaws.

Accuracy of Vernier Calliper:

The accuracy of a vernier caliper is 0.1 mm or 0.01 cm. This means that the smallest distance that a vernier caliper can measure is 0.1 mm or 0.01 cm.

### How to Read Vernier Calliper:

Place the object between the jaws of the caliper and close the jaws until the object is held firmly.

Read the main scale reading before the zero (0) on the vernier scale. Find the markings on the vernier scale that coincides with the mark on the main scale.

Multiply the reading of the vernier scale by the accuracy of the vernier scale which is 0.01 cm

Example 1

In the figure above, find the location on the main scale, that is before the 0 mark of the vernier scale. In the figure, this would be between 6.10cm.

Find the mark on the vernier scale that most closely aligns with any mark on the main scale. Here 6 lines up best with one of the marks on the main scale.

Multiply the reading of the vernier scale by the accuracy of the vernier scale which is 0.01 cm.

= 6 x 0.01 cm  = 0.06 cm

6.10 cm + 0.06 cm = 6.16 cm

Example 2

Main scale reading: 10.0 cm (Immediate left of zero)

Vernier scale reading: 2 (Alignment of scale lines)

Reading of vernier scale x accuracy of vernier scale

2 x 0.01 cm = 0.02 cm

= 10.02cm

Example 3

In this example, the vernier scale is in mm. Remember the accuracy of a vernier caliper is 0.1 mm or 0.01cm. We will be using 0.1 mm in this example.

In the figure above, find the location on the main scale, that is before the 0 mark of the vernier scale. In the figure, this would be between 3.70 cm or 37mm.

Find the mark on the vernier scale that most closely aligns with any mark on the main scale. Here 4.6 lines up best with one of the marks on the main scale. The scale is in mm.

Multiply the reading of the vernier scale by the accuracy of the vernier scale which is 0.1 mm.

= 4.6 x 0.1 mm  = 0.46 mm

37 mm + 0.46 mm = 37.46 mm

### Micrometer Screw Gauge:

This is used for short readings between 0 and 2.5 cm. The reading accuracy of the instrument is 0.001 cm or 0.1 mm. A micrometer has two scales; main scale (on the sleeve) and the circular scale/vernier scale (on the thimble).

Each division on the main scale is 1 mm and the division on the vernier scale is 0.01 mm.

### How to Read Micrometer Screw Gauge:

• Turn the thimble until the object is firmly gripped.
• Read the line on the circular scale that coincides with the line on the main scale.
• Multiply the reading of the vernier scale by the accuracy of the vernier scale which is 0.01mm.
• Finally, add the reading of the main scale and the vernier scale together to obtain results in mm.

Example 1

Main scale (sleeve) reading: 3.50 mm

Reading of vernier scale x accuracy of vernier scale

= 27 x 0.01 mm

= 0.27 mm

Required reading: 3.50 mm + 0.27 mm = 3.77 mm

Example 2

Main scale (sleeve) reading: 7.00 mm 