The enlarged scale in figure 2-38 can help youunderstand how to take a complete micrometer read-ing to the nearest thousandth of an inch.As you can see, the thimble has been turnedthrough 12 spaces of its scale, and the 12th graduationis lined up with the reference line on the sleeve. Whenthe value on the sleeve scale is added to the value onthe thimble scale that is lined up with the referenceline on the sleeve scale, the space between the anviland spindle must be 0.712 inch (seven hundred andtwelve-thousandths of an inch).Occasionally you attain a reading in which thehorizontal reference line of the sleeve scale falls be-tween two graduations on the thimble scale, as shownin figure 2-39. Note the horizontal reference line iscloser to the 15 mark than the 14 mark. To read thismeasurement to THREE decimal places, simplyround off to the 15 mark as shown in example A offigure 2-39. To read this measurement to FOUR deci-mal places, estimate the number of tenths of the dis-tance between thimble scale graduations thehorizontal reference line has fallen. Each tenth of thisdistance equals one ten-thousandth (0.0001) of aninch. Add the ten-thousandths to the reading as shownin example B of figure 2-39.Reading the Vernier Scaleon a MicrometerMany times you are required to work to excep-tionally precise dimensions. Under these conditions itis better to use a micrometer that is accurate to ten-thousandths of an inch. This degree of accuracy isobtained by the addition of a vernier scale.The vernier scale of a micrometer (fig. 2-40) fur-nishes the fine readings between the lines on theFigure 2-38.—Enlarged micrometer scale.Figure 2-39.—Estimating a micrometer reading.thimble rather than requiring you to interpolate (orestimate) the reading. The 10 spaces on the vernier areequivalent to 9 spaces on the thimble. Therefore, eachunit on the vernier scale is equal to 0.0009 inch andthe difference between the sizes of the units on eachscale is 0.0001 inch.When a line on the thimble scale does not coin-cide with the horizontal reference line on the sleeve,you can determine the additional spaces beyond thereadable thimble mark by finding which vernier markmatches up with a line on the thimble scale. Add thisFigure 2-40.—Vernier scale of a micrometer.2-26