Inventing for Boys

by A. Frederick Collins

The micrometer is a tool, or instrument, which will measure accurately from 0 to 1 inch in thousandths of an inch. It is one of the most useful measuring devices that has ever been invented and if you are to build a model accurately you cannot get along without one. It is shown in Fig. 72.

There are five parts to a micrometer and these are (a) the frame; (b) the anvil; (c) the spindle; (d) the sleeve and (e) the thimble. The frame is held in the left hand, the object to be measured being placed between the anvil and the spindle; the thimble is turned by the thumb and finger of your right hand and the spindle, which is fastened to the thimble, turns with it and moves through the nut in the frame until the end of the spindle touches the object to be measured.

Fig. 72. A MICROMETER FOR MEASURING THOUSANDTHS OF AN INCH

The measurement of the object is shown by the vertical lines on the spindle and the horizontal lines on the thimble and both of which are numbered. They are really a form of vernier.

To read the micrometer, that is to find the measurement of an object, you have only to multiply the number of vertical divisions which you can see on the sleeve by 25 and to this add the number of divisions on the bevel of the thimble from the 0 line to the line which coincides with it, that is, comes even with the long horizontal line on the sleeve. It is easy to learn to read a micrometer by taking one in your hands, making a few measurements and following the above instructions.

The wire gage is a circular piece of flat steel a little larger than a silver dollar and it is used to find the numbers of, and to measure the sizes of, wires. There are 32 slots cut in the edge each ending in a hole and numbered from 5 to 36 as shown in Fig. 73.

Fig. 73. A STANDARD WIRE GAGE

To find the number of a certain sized wire slip the latter into the slots until you find one into which it will just pass snugly and the number of the slot will be the number of the wire. On the reverse side of the gage will be found the sizes of the wire in decimal fractions of an inch.