The ratio of one number to another is the quotient
of the first, divided by the second. This is often ex-
pressed as a:b, which is read the ratio of a to b. More
commonly, this is expressed as the fraction a/b.
Ratio has no meaning unless both terms are ex-
pressed in the same unit by measurement.
What is the ratio of the diameter of circle O to circle
M? This ratio is D:d or D/d. If the diameter of O is 3
inches and the diameter of M is 1.5 inches, then the ratio
of the diameters of circle O and circle M would be 3/1.5
or 2/1 (read ratio of two 1 to one).
What is the ratio of the diameter of circle M to the
diameter of circle O?
RATIO APPLIED TO SCALE DRAWINGS
Since it is not always possible to make a drawing
full size, the size of the drawing maybe made in a given
ratio to the full size of the object.
In the above example, A represents the object in its
full size and B represents a drawing one-half size. The
ratio of the drawing B to object A is one to two. (1/2)
Drawings may be commonly scaled down by
use of the following ratios:
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)
The above illustration shows a portion of a circum-
ference rule. This is an example of the application of a
ratio. The upper edge of the rule is graduated in such a
manner that one inch on the upper scale is in the ratio of
3.1416 to 1 on the lower scale. This is in the ratio of the
circumference of a circle to its diameter, so that any
diameter can be converted to a circumference or vice
versa by reading directly across the rule.
In sheet metal pattern development, effective use
can be made of the circumference rule. By using the
circumference side, you can lay out the development of
large objects. After making the layout, you can make the
development of the pattern full size.
Percentage (%) is a way of expressing the relation-
ship of one number to another. In reality, percentage is
a ratio expressed as a fraction in which the denominator
is always one hundred.
The ratio of 6 to 12, expressed as %:
The ratio of 6 to 12 may be expressed as .5 or 1. To
change to %, move the decimal two places to the right.
This means there are 50 parts to 100.
From a galvanized iron sheet weighing 46 1/4
pounds, an "ell" and one section of pipe were produced