RATIOThe ratio of one number to another is the quotientof the first, divided by the second. This is often ex-pressed as a:b, which is read the ratio of a to b. Morecommonly, this is expressed as the fraction a/b.Ratio has no meaning unless both terms are ex-pressed in the same unit by measurement.Example:What is the ratio of the diameter of circle O to circleM? This ratio is D:d or D/d. If the diameter of O is 3inches and the diameter of M is 1.5 inches, then the ratioof the diameters of circle O and circle M would be 3/1.5or 2/1 (read “ratio of two 1 to one”).What is the ratio of the diameter of circle M to thediameter of circle O?RATIO APPLIED TO SCALE DRAWINGSSince it is not always possible to make a drawingfull size, the size of the drawing maybe made in a givenratio to the full size of the object.Example:In the above example, A represents the object in itsfull size and B represents a drawing one-half size. Theratio of the drawing B to object A is one to two. (1/2)(1:2)Drawings may be commonly “scaled down” byuse of the following ratios:theLet:1/4 = 1'0" or a ratio of (1/48) or (1 to 48)1/8” = 1'0" or a ratio of (1/96) or (1 to 96)3 = 1'0" or a ratio of (1/4) or (1 to 4)3/8 = 1'0" or a ratio of (1/32) or (1 to 32)CIRCUMFERENCE RULEThe above illustration shows a portion of a circum-ference rule. This is an example of the application of aratio. The upper edge of the rule is graduated in such amanner that one inch on the upper scale is in the ratio of3.1416 to 1 on the lower scale. This is in the ratio of thecircumference of a circle to its diameter, so that anydiameter can be converted to a circumference or viceversa by reading directly across the rule.In sheet metal pattern development, effective usecan be made of the circumference rule. By using thecircumference side, you can lay out the development oflarge objects. After making the layout, you can make thedevelopment of the pattern full size.PERCENTAGEPercentage (%) is a way of expressing the relation-ship of one number to another. In reality, percentage isa ratio expressed as a fraction in which the denominatoris always one hundred.Example:The ratio of 6 to 12, expressed as %:The ratio of 6 to 12 may be expressed as .5 or 1. Tochange to %, move the decimal two places to the right.This means there are 50 parts to 100.From a galvanized iron sheet weighing 46 1/4pounds, an "ell" and one section of pipe were producedAII-21

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