# Measurement of Length

1. Length is the distance between two points. The SI unit of length is metre (m).
2. 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.

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

### Measuring Length of a Curve.

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

#### Using an opisometer ( map measurer )

1. An opisometer, also called a curvimeter, is an instrument for measuring the lengths of curved lines.
2. A simple opisometer consists of a toothed wheel of known circumference on a handle.
3. 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.

1. Figure above shows how to measure diameter of a Ping-Pong ball.
2. 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)
1. A vernier caliper is used to measure small distance/length of objects.
2. The external jaws of a vernier caliper is for measuring external diameter while the internal jaws are for measuring internal diameter.
3. The stem is for measuring depth of narrow holes.
4. A vernier caliper has two scales.
1. A main scale with smallest division of 0.1 cm.
2. A vernier scale marked from 0 to 10.
5. Readings from a vernier caliper are expressed to two decimal places of a centimetre.
6. Videos below explain how reading is taken from a vernier caliper.

1. 