The sleeve is marked with 40 lines to the inch, corresponding to the number of threads on the spindle. When the spindle is down against the anvil, the beveled edge of the thimble coincides with the lone 0 on the sleeve, and the 0 line on the thimble coincides with the horizontal line on the sleeve. By turning the knurled thimble with the thumb and finger until the 0 line on the thimble again agrees with the horizontal line on the sleeve, the distance between the anvil and the bottom point of the spindle will be 1/40, or .025 of an inch, and the beveled edge of the thimble will coincide with the second vertical line on the sleeve. Each vertical line on the sleeve indicates a distance of 1/40, or .025 of an inch. Every fourth line on the sleeve is made longer than the others, and is numbered 0, 1, 2, 3, 4, etc., up to 0 or the capacity of the micrometer. Each numbered line indicates a distance of four times 1/40 of an inch, or 1/10.
The beveled edge of the thimble is marked in twenty-five divisions, and every fifth line is numbered from 0 to 25. Turning the spindle from one of these marks to the next indicates that the spindle has been moved 1/25 of .025, or one-thousandth of an inch.
Hold the frame stationary and revolve the thimble with the thumb and finger. The spindle, being attached to the thimble, revolves with it, and moves through the nut in the frame, approaching or receding from the anvil. The measurement of the opening between the anvil and the spindle is shown by the lines and figures on the sleeve and the thimble.
To read the micrometer, place the object to be measured on the anvil, turning the thimble up or down until it touches the object lightly. Multiply the amount of vertical divisions visible on the sleeve by 25 and add the number of divisions on the bevel of the thimble from 0 to the line which coincides with the horizontal line on the sleeve.
For example, if there are 5 divisions visible on the sleeve and six lines showing on the thimble, multiply 5 by 25, and add 6. Total .131 of an inch.
THE POINT SYSTEM
Some time prior to the year 1450 Gutenberg invented the casting of metal type in molds. As the art of printing advanced, many new sizes were cast, but no attempt was made to cast them with a uniform gradation in size and it was difficult to build up one size of body to equal another; that is, justify them.
To obviate this, Fournier, in 1737, advocated a method of casting type according to some unit. The size known as pica was in use in various countries in Europe, and was considered a standard size. Taking the pica as a basis he divided it into twelve parts, each of which he called a point. He chose one-twelfth of a pica as the unit because there existed five sizes of type between pica and nonpareil. As nonpareil was just half the size of pica, this made the succession of sizes seven, eight, nine, ten, and eleven points, any of which could be justified with another by the use of material made to the same unit.
The United States Typefounders’ Association finally adopted it in 1887. It is the only system in use in first-class offices today.
It is popularly supposed that six picas equal one inch. This is approximately so, but not absolutely, for six picas measure but .99648 of an inch. The American pica runs about three points less than 72 lines to the foot. Its actual measurement is .16608 of an inch. One-twelfth of this, or one point, is, therefore .01384 of an inch.