# Protractor

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A protractor is a measuring instrument, typically made of transparent plastic or glass, for measuring angles.

## Contents

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

## Typical protractor

Most protractors measure angles in degrees (°). Radian-scale protractors measure angles in radians. Most protractors are divided into 180 equal parts. Some precision protractors further divide degrees into arcminutes.

They are used for a variety of mechanical and engineering-related applications. One common use is in geometry lessons in schools.

## Bevel protractor

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

Universal bevel protractors are also used by toolmakers; as they measure angles by mechanical contact they are classed as mechanical protractors. [1] [2]

The bevel protractor is used to establish and test angles to very close tolerances. It reads to 5 arcminutes (5′ or 1/12°) 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.

## Related Research Articles

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A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation.

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A theodolite is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but they are also used extensively for building and infrastructure construction, and some specialized applications such as meteorology and rocket launching.

A torque wrench is a tool used to apply a specific torque to a fastener such as a nut, bolt, or lag screw. It is usually in the form of a socket wrench with special internal mechanisms.

A combination square is a multi-purpose measuring and marking tool used in metalworking, woodworking, and stonemasonry. It is composed of a rule and one or more interchangeable heads that can be attached to the rule.

The steel square is a tool used in carpentry. Carpenters use various tools to lay out structures that are square, many of which are made of steel, but the name steel square refers to a specific long-armed square that has additional uses for measurement, especially of various angles. Today the steel square is more commonly referred to as the framing square or carpenter's square. It consists of a long, wider arm and a shorter, narrower arm, which meet at an angle of 90 degrees. It can also be made of aluminum or polymers, which are light and resistant to rust.

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A caliper is a device used to measure the dimensions of an object.

A machinist square or engineer's square is the metalworkers' equivalent of a try square. It consists of a steel blade inserted and either welded or pinned into a heavier body at an angle of 90°. Usually a small notch is present at the inside corner of the square. This prevents small particles from accumulating at the juncture and affecting the square's reading.

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A set square or triangle is an object used in engineering and technical drawing, with the aim of providing a straightedge at a right angle or other particular planar angle to a baseline.

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A dividing engine is a device employed to mark graduations on measuring instruments to allow for reading smaller measurements than can be allowed by directly engraving them. The well-known vernier scale and micrometer screw-gauge are classic examples that make use of such graduations.

Madhava's sine table is the table of trigonometric sines of various angles constructed by the 14th century Kerala mathematician-astronomer Madhava of Sangamagrama. The table lists the trigonometric sines of the twenty-four angles 3.75°, 7.50°, 11.25°, ..., and 90.00°. The table is encoded in the letters of Devanagari using the Katapayadi system. This gives the entries in the table an appearance of the verses of a poem in Sanskrit.

Vertical declining dials are sundials that indicate local apparent time. Vertical south dials are a special case: as are vertical north, vertical east and vertical west dials. The word declining means that the wall is offset from one of these 4 cardinal points. There are dials that are not vertical, and these are called reclining dials.

## References

1. Capotosto, Rosario (December 1993). "Bridge City Protractor". Tool Test. Popular Mechanics . p. 76. ISSN   0032-4558 . Retrieved 12 April 2020.
2. Farago, Francis T; Curtis (1994). Handbook of Dimensional Measurement. Industrial Press Inc. p. 580. ISBN   0-8311-3053-9.