The first micrometer was invented by William Gascoigne in the 17th century to assist in measuring angular distances between stars while using a telescope. Jean Laurent Palmer of France developed the tool further in 1848 making it possible to use to measure hand held devices and therefore was originally named after him, the palmer, as it is still known today. In Spain it is still known as the palmer screw or tomillo de palmer.
By the late 1800′s the micrometer had been mass marketed into many machine shops throughout these United States. Though it’s form has changed many times and the objects it can measure can differ such as outside and inside diameters, and depth, the particular micrometer we will discuss is the one that measures in inches the outside diameter or the micrometer caliper.
The micrometer or “mike” can measure the diameter of an object within 0.001 inches. Some micrometers have an vernier added to them that allow the measurements to done at 0.0001. No matter the size of the frame of a micrometer, and some can get pretty large, the actual maximum area that can be measured is only 1 inch total.
The micrometer consists of the anvil (the fixed part of the measuring area), the frame, the spindle (the part that moves while measuring), the spindle lock (that locks the caliper in place after measurement in the event you are using it to compare parts or such), the barrel which has the quarter inch (0.25) lines, the sleeve, sometimes called thimble, which contains the lines for another quarter inch divided into 40 lines representing the 0.001, and the ratchet to use for fine tuning measurements.
“The scale of a micrometer ranges from 0 to 1 inch and is usually graduated in thousandths of an inch (think of 1 in. as 1000/1000). The sleeve of the mike (as it slowly pulls back) shows the numbers 0 1 2 3 4 5 6 7 8 9 0. Each of these numbers represents 0.100 (or 100 thousandths) of an inch…Now look at the thimble. Each line on the thimble represents 0.001, and there are number every five lines (or 0.005). One complete turn (revolution) of the thimble is equal to 0.025 in. So each revolution of the thimble is equal to one division on the barrel.” (Griffith, 2003)
The Stefanelli website has a great simulator of a inch micrometer that you can move with the mouse and it will tell you the readings while you practice. I would also suggest you find some old machine tool textbooks and look for exercises that will test you in reading the measurements. As I have said before, know your tool or measuring device before using it in your actual day to day use. Refer to the previous post for more general suggestions on measuring.