Math Doubts

Protractor

Definition

A geometric tool that contains a scale marked from zero to one hundred eighty degrees is called a protractor.

What is a Protractor?

Once you know the concept of angle in geometry, a standard tool is required for us to measure the angle physically. So, a basic geometric instrument, called protractor, was invented to measure the angles geometrically.

protractor

The protractor is made-up of plastic or metal. It is available in two shapes; which means semicircular shape and circular shape, a semicircular protractor is popularly used while we learn how to measure the angles by a protractor in mathematics. The above diagram shows the visual example of a protractor in semicircular shape.

Parts of a Protractor

There are three major parts in a half-circle protractor and let’s begin to know its each part.

Baseline

Look at the picture of protractor, there is a straight line in horizontal direction at the bottom of a protractor, it acts as a reference line to set the base of measuring an angle geometrically and it is called the baseline of protractor.

baseline of protractor

The baseline of a protractor is bisected by a vertical line at its middle point. So, the baseline is divided into two equal parts in a protractor and they are called by the following names in geometry.

  1. Left-side baseline
  2. Right-side baseline

Now, let’s know more about another part of a protractor clearly from understandable explanation.

Origin

Look at the below image of protractor, the baseline is perpendicularly bisected by a vertical-line at its middle point and their intersecting point is called the origin of protractor.

origin of protractor

At the bottom of a protractor, the baseline is exactly bisected at its middle point by a vertical line. Hence, the origin of the protractor is also called by the following three other names.

  1. Vertex
  2. Center or Centre
  3. Midpoint

The origin of a protractor can be called by any name but you should understand that it is a middle point of baseline and also the intersecting point of the baseline and vertical line in a protractor.

Degree scale

There is a scale in degrees circularly on a protractor and it has three notable factors.

  1. Degree marks
  2. Inner scale or Anticlockwise degree scale
  3. Outer scale or Clockwise degree scale

Now, let’s learn about them in detail with understandable graphical representations.

Degree marks

The semicircular protractor is divided into one hundred eighty ($180$) equal parts from its origin. So, each division denotes one degree ($1^\circ$) and a semicircular protractor denotes one hundred eighty degrees ($180^\circ$) geometrically.

degree marks of protractor

The circular scale in degrees on the protractor is displayed in both clockwise and anticlockwise directions.

Inner degree scale

At right side, the circular scale is denoted by the numbers from $0$ to $180$ degrees in anticlockwise direction. It is actually displayed below to another scale. So, it is called the inner degree scale of protractor and it is also called the anticlockwise degree scale of a protractor.

inner and outer degree scale of protractor

The inner degree scale on a protractor is mainly used to measure the angle in mathematics.

Outer degree scale

At left side, the circular scale is denoted by the numbers from $0$ to $180$ degrees in clockwise direction. It is actually displayed above to another scale. So, it is called the outer degree scale of protractor and it is also called the clockwise degree scale of a protractor.