fourth graduation is numbered 1, 2, 3, 4, and so
on, representing 0.100 inch, 0.200 inch, and so
on. When you turn the thimble so that its edge
is over the first sleeve line past the 0 on the
thimble scale, the spindle has opened 0.025 inch.
If you turn the spindle to the second mark, it has
moved 0.025 inch plus 0.025 inch or 0.050 inch.
You use the scale on the thimble to complete your
reading when the edge of the thimble stops
between graduated lines. This scale is divided
into 25 equal parts, each part representing 1/25
of a turn; and 1/25 of 0.025 inch is 0.001 inch.
As you can see, every fifth line on the thimble
scale is marked 5, 10, 15, and so on. The thimble
scale, therefore, permits you to take very accurate
readings to the thousandths of an inch; since you
can estimate between the divisions on the
thimble scale, fairly accurate readings to the ten
thousandth of an inch are possible.
The closeup in figure 3-22 helps you to under-
Count the units on the thimble scale and add them
to the reading on the sleeve scale. The reading in
the figure shows a sleeve reading of 0.250 inch
(the thimble having stopped slightly more than
Figure 3-21.--Sleeve and thimble scales of a micrometer
halfway between 2 and 3 on the sleeve) with the
10th line on the thimble scale coinciding with the
horizontal sleeve line. Number 10 on this scale