Protractor Protractor A protractor is a measuring instrument, typically made of transparent plastic or glass, for measuring angles. 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. 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. A bevel protractor is a graduated
Circular surface "I" → R is called the base curve for the circular surface and the two maps u, v : "I" → R are called the direction frame for the circular surface. For a fixed "t" ∈ "I" the image of "ƒ"("t", "θ") is called a generating circle of the circular surface. Circular surfaces are an analogue of ruled surfaces. In the case of circular surfaces the generators are circles; called the generating circles. In the case of ruled surface the generators are straight lines; called rulings. Circular surface In mathematics and, in particular, differential geometry a circular surface is the