Protractor 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. Reading the Vernier scale: The bevel protractor Vernier scale may have graduations of 5′ (minutes) or 1/12°. Each space on the Vernier scale is 5′ less
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