Reading in thousandths. As stated, the micrometer screw has usually forty threads per inch and the thimble has twenty-five divisions on its circumference. The barrel is divided to correspond to the pitch of the screw with each fourth division numbered. In reading the indicated measurement, first note the highest number visible on the barrel and call it hundreds of thousands-in Fig. 21 it is 400 thousandths or .400; then read the short divisions on the barrel, calling the first division 25 thousandths, or .025; the second, 50 thousandths, or .050; and the third, 75 thousandths, or .075. In Fig. 21 the third division is the last one visible. Now read the number indicated on the thimble, that is, the number that has passed the line running lengthwise. In the figure it is 16; or 16 1/2 if the reading is to be finer than thousandths. Add this reading to the readings of the short divisions, thus: 75+ 161/2 = 91 1/2; this is .091 1/2. Adding the .400 to this we get .491 1/2.This means that the distance from the anvil to the measuring point is 4915/10000 of an inch, or .4915 inch. If the micrometer caliper is a good one, we may be sure the distance is between .491 inch and .492 inch.

Fig. 22. Inside Micrometers Courtesy of L. S. Starrett Company, Athol, Massachusetts.