Protractors

A protractor is a mathematical instrument made from clear plastic used for measuring angles in degrees such as the angles of a triangle or a hexagon.

Types of Protractor

• Semi-Circular Protractor: This protractor has two sets of scales graduated up to 180° in opposite directions enabling angles to be read from two directions. On the base of the protractor is a midpoint mark that indicates the position of the highest point of the angle that is going to be measured or the vertex. The semi-circular protractor is used for reading, drawing and plotting angles on paper or charts. This type of protractor is commonly used in classrooms for geometry and in mechanical and engineering-related applications.

• The Vernier Scale: This is used on protractors to enable them to give more precise and accurate measurements. There are 60 minutes in a degree and regular protractors have a scale graduated in degrees. In the Vernier scale, each graduation represents 5 minutes or 1/12 of a degree. The Vernier scale is found underneath the main degree scale and is spaced so it reads at a constant fraction of the fixed main scale.

• Bevel Protractors: These are able to measure through 360° by using one or two swinging arms that can help measure the angle. The construction of a bevel protractor consists of a beam, graduated dial and a blade which is connected to a swivel plate by thumb nut and clamp. There are variants on this design including a universal precision protractor and precision universal Vernier protractor which have a more technical design. Bevel protractors are more accurate than regular protractors as they use a Vernier scale along with a fixed scale. The fixed dial or main scale has four sections with each one graduated from 0 ° to 90 °. The Vernier scale has 24 divisions with 12 divisions on either side of 0 with each division representing 5 minutes. As with the regular protractor the Vernier scale is bidirectional and can be read to either side. Bevel protractors are designed for making for high precision measurements and are commonly used for producing mechanical and architectural drawings and by tool makers. Bevel protractors are often ground and made from stainless steel. Some protractors have a satin chrome finish to improve visibility of the scales.