A protractor is a

measuring instrument A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Est ...

, typically made of transparent plastic or glass, for measuring

angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...

s. Some protractors are simple half-discs or full circles. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle.

Typical

Most protractors measure angles in degrees (°). Radian-scale protractors measure angles in

radians The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before that ...

. Most protractors are divided into 180 equal parts. Some precision protractors further divide degrees into

arcminutes A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...

. They are used in mechanics, engineering and geometry.

Bevel

A bevel protractor is a graduated circular protractor with one pivoted arm; used for measuring or marking off angles. Sometimes

Vernier scale A vernier scale, named after Pierre Vernier, is a visual aid to take an accurate measurement reading between two graduation markings on a linear scale by using mechanical interpolation, thereby increasing resolution and reducing measurement unce ...

s are attached to give more precise readings. It has wide application in architectural and mechanical drawing, although its use is decreasing with the availability of modern drawing software or

CAD Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...

. Universal bevel protractors are also used by toolmakers; as they measure angles by mechanical contact they are classed as mechanical protractors. The bevel protractor is used to establish and test angles to very close tolerances. It reads to 5 arcminutes (5′ or °) and can measure angles from 0° to 360°. The bevel protractor consists of a beam, a graduated dial and a blade which is connected to a swivel plate (with Vernier scale) by thumb nut and clamp. When the edges of the beam and blade are parallel, a small mark on the swivel plate coincides with the zero line on the graduated dial. To measure an angle between the beam and the blade of 90° or less, the reading may be obtained direct from the graduation number on the dial indicated by the mark on the swivel plate. To measure an angle of over 90°, subtract the number of degrees as indicated on the dial from 180°, as the dial is graduated from opposite zero marks to 90° each way. Since the spaces, both on the main scale and the Vernier scale, are numbered both to the right and to the left from zero, any angle can be measured. The readings can be taken either to the right or to the left, according to the direction in which the zero on the main scale is moved.

Gallery

Image:Protractor1.svg, A 360° protractor marked in degrees. Image:Grad protractor.png, A 400 gon protractor marked in gradians. Image:Navigational rules types.JPG, A " Cras navigation plotter" double-protractor, in foreground, named after his inventor Jean Cras. Image:Protractor_Rapporteur_Degrees_V3.jpg, A half circle protractor marked in degrees (180°). Image:Set square Geodreieck.svg, , a special type of protractor triangle (180°). File:Electronic protractor.jpg, Electronic protractor

References

{{Authority control Dimensional instruments Mathematical tools Navigational equipment Technical drawing tools Angle measuring instruments.

Typical

Bevel

Gallery

See also

References