Plain Scale

PLAIN SCALE

A plain scale is the simplest scale used to represent two consecutive units, ie., a unit and its sub-division.

Examples : 

1. Metre and decimetre (1 metre = 10 decimetre) 

2. Kilometre and hectometre (1 kilometre = 10 hectometre)

3. Decimetre and Centimetre (1 decimetre = 10 centimetre)

4. Feet and inches (1 foot = 12 inches)

Both First unit, (ie., main unit) and Second unit (ie., subunit) are measured along the length of scale but the second unit of the scale is comfortably marked such that the first main division is subdivided into equal number of subunits.

The following steps are followed for the construction of a plain scale. 

1. Determine the main and subunits of the scale. 

2. Determine the length of scale,

Ls = R.F × Maximum length of object 

(If R.F is not given in the problem, it can be assumed suitably) 

3. Draw a rectangle of length Ls and width 5 to 10 mm approximately. 

4. Divide Ls into divisions such that each division represents the main unit. 

5. Divide the first main division into appropriate number of equal parts so that each division will represent the subunit. 

6. Mark O at the end of the first main division. 

7. Label the main divisions of the scale from `O' mark towards the right and label the subdivisions of the scale from `O' mark towards the left. 

8. Write the name of main unit, subunit and R.F value below the scale. 

9. Mark the required length on the scale thus prepared.

Example 4 : Construct a plain scale of R.F = 1 : 40 to read metres and decimetres and long enough to measure upto 5 m. Mark a dimension on the scale representing 3.7 m. 

Data given : R.F = 1/40 ; Maximum length = 5 m

Main unit and Sub unit : Metre and Decimetre. 

1. Length of scale, Ls = R.F × Maximum length

= 1 / 40 × 5 m

= 1 / 40 × 5 × 100 cm

= 12.5 cm

2. Draw a rectangle having length AB = 12.5 cm and width 10 mm (Assumed) 

3. Divide the length of scale (ie. 12.5 cm) into 5 equal parts so that each part will represent 1 metre. 

4. Mark O at the end of first main division, and the other divisions as 1, 2, 3 and 4 towards right. 

5. Divide the first main division into 10 equal parts so that each subdivision will represent 1 decimetre. Mark the subdivisions as 1, 2, 3, etc from O towards left. 

6. Mark a length of 3.7 m on the scale, ie., 3 metre on the right of the zero mark and 7 decimetre on the left side of zero mark.

The scale thus constructed is shown in Fig. 5.3.