Notice that there are 9 "tick marks" (Figure 4.3-6) between each whole number from 10 to
15. Since the tick marks make a total of ten divisions between the whole numbers, each
tick mark represents a difference of one. Because of the floating decimal, the first mark to
the right of ten could represent 10.1, 101, or 1010. There are 4 tick marks between each
whole number from 15 to 30. In this case, each tick mark represents a difference of two,
therefore the first unmarked value to the right of 15 could represent 15.2, 152, or 1520.
There is a single tick mark between the whole numbers between 30 and 60 with each
representing a difference of five. The first unmarked value after the 30 could represent
30.5, 305, or 3050.
When it is necessary to read an unmarked value between two of the marked divisions,
determine the values of the tick marks and interpolate. The value 151 would be found
halfway between 15 and the first tick mark past 15. 307 would be slightly less than half way
between the first mark past 30 and the next large mark.
Figure 4.3-6 Tick Marks
This index will be used for most problems that involve time. Note that this mark is found
where the 60 would normally be on the inner wheel. It is used for any problem where the
unit of time being considered is an hour. (Figure 4.3-7)