- Length is the distance between two points. The SI unit of length is metre (m).
- Length is usually measured in kilometre (km), centimetre (cm) and millimetre (mm).

1 km = 1,000 m

1 m = 100 cm or

1 m = 1,000 mm

1 cm = 10 mm

### Measuring Length of a Straight Line.

- The length of a straight line is usually measure by a ruler.
- In order to obtain an accurate measurement, the following steps need to be taken.
- When reading the measurement, eye must be vertically above the mark on the ruler to avoid parallax errors.
- Measurements are repeated to get an average length.

### Measuring Length of a Curve.

#### Using thread and ruler

- The length of a curve can be measured by using a thread and a ruler.
- The string is placed along the curve and the length is marked.
- After that, straighten the string along the side of a ruler to measure its length.
- You are advised to repeat the measurement a few times to get the average reading.

#### Using an opisometer ( map measurer )

- An opisometer, also called a curvimeter, is an instrument for measuring the lengths of curved lines.
- A simple opisometer consists of a toothed wheel of known circumference on a handle.
- Image below shows an opisometer used to measure the length of a curve on a map.

(Opisometer. This image is shared by Frank C. Müller at Wikipedia) |

### Measuring Diameter of an Object.

- Figure above shows how to measure diameter of a Ping-Pong ball.
- In labaoratory, usally an external caliper and a ruler are used to measure external diameter of objects while an internal caliper and a ruler are used to measure internal diameter of objects.

### The Vernier Caliper

(Vernier Caliper) |

- A vernier caliper is used to measure small distance/length of objects.
- The external jaws of a vernier caliper is for measuring external diameter while the internal jaws are for measuring internal diameter.
- The stem is for measuring depth of narrow holes.
- A vernier caliper has two scales.
- A main scale with smallest division of 0.1 cm.
- A vernier scale marked from 0 to 10.
- Readings from a vernier caliper are expressed to two decimal places of a centimetre.
- Videos below explain how reading is taken from a vernier caliper.

**Youtube Video 1**

**Youtube Video 2**

External Link

Uni. Cape Town: Vernier Calliper

Interactive Animation

NTNU: Vernier Caliper (Java Applet)

SHAW : Vernier Caliper (Java Applet)

Uni. Cape Town: Vernier Calliper

Interactive Animation

NTNU: Vernier Caliper (Java Applet)

SHAW : Vernier Caliper (Java Applet)

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