Get a common iron or brass bolt about 1/4 of an inch in diameter and about 2 1/2 inches long, with as fine a thread as possible, and the thread cut to within a short distance of the head of the bolt. A bolt with a cut in the head for a screw-driver should be used. Clamp together two blocks of wood with square corners about 1 inch wide, 3/4 of an inch thick, and 2 1/2 inches long, with their narrower faces in contact (the width of the clamped blocks being 2 inches), and bore a 1/4-inch hole through the center of the blocks in the 2-inch direction. Now remove the clamp, and let the nut of the bolt into one of the blocks so that its hole will be continuous with the hole in the wood, then glue the blocks together with the nut between them. Cut out a piece from the block combination, leaving it shaped somewhat like a bench, and glue the bottoms of the legs to a piece of thin board about 2 1/2 inches square for a support. Solder one end of a stiff wire about 2 inches long to the head of the bolt at right angles to the shaft, and fix a disk of heavy pasteboard with a radius equal to the length of the wire, and with its circumference graduated into equal spaces, to serve in measuring revolutions and parts of revolutions of the end of the wire, to the top of the bench; put the bolt in the hole, screwing it through the nut, and the construction is complete.

Fig. 123 - A home-made micrometer.

The base is improved for the measuring work by gluing to a central section of it, covering the place where the end of the bolt meets it, a small piece of stiff metal; and it is convenient to have the graduated disk capable of rotating, so that its zero line may be made to coincide with the wire.

Find the number of threads of the screw to the inch by placing the bolt on a measuring rule, and counting the threads in an inch or half an inch of its length. The bolt in making one revolution will descend a distance equal to the distance between the threads.

To use the apparatus, put the object whose thickness is to be measured on the base under the bolt, and screw the bolt down until its end just touches the object, then remove the object and screw the bolt down until its end just touches the base, carefully noting while doing so the distance that the end of the wire moves-over the scale. The part of a rotation of the bolt, or the number of rotations with any additional parts of a rotation added, divided by the number of threads to the inch, will be the thickness of the object. Quite accurate measurements may be made with this instrument, and in the absence of the expensive micrometer, it serves a very useful purpose.