A protractor measures angles between intersecting lines in degrees, with a basic semicircular design measuring up to 180 degrees. Other types include a two-straight-edge design and a circular design for measuring all angles. Protractors can also be used to draw lines at arbitrary angles.
A protractor is a device for measuring the angle between two intersecting lines. Angle is measured in degrees, and a circle is defined as having 360 degrees of identical size. The first known protractors were designed in the 17th century by mariners who needed to plot courses on nautical charts.
A basic protractor consists of a semi-circular piece of material with degree marks engraved on the outer edge of the arc or curved section. The ruler is placed along one of the lines to be measured and the number of degrees is determined by where the second line crosses the arc.
Because it’s a semicircle, the basic protractor only measures angles up to 180 degrees. This is adequate in normal use, since the relationship between two lines can always be expressed as an angle of 180 degrees or less. If the original placement of the protractor produces an angle greater than 180 degrees, simply flipping it over will give a reading of less than 180 degrees.
This is due to the symmetrical nature of the circle; starting with one line, the angle to a second intersecting line can be measured moving clockwise or counterclockwise. One direction will measure an angle greater than 180 degrees and the other will measure an angle less than 180 degrees. Both cannot measure more than 180, as they must add up to 360, the number of degrees in a full circle.
An alternative type of protractor consists of two straight edges attached to each other at one end, with a graduated arc between them. The bow is attached to one arm and the other arm slides freely over it. One arm is placed along a line and the other arm is rotated outward until it is parallel to the other line. The degrees are then read from the arc. Astronomers often use a third type of protractor, made from a completely circular piece of material. This allows them to directly measure all angles between 0 and 360.
Protractors are typically used to measure existing lines, but can also be constructed to help draw them. By attaching a swivel arm to the straight edge of the semicircular disc described above, lines at arbitrary angles can be drawn. Such devices are most commonly used in drafting technical drawings such as architectural plans.
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