The Mechanical Pencil: Feeding a Thin Lead by a Screw

Keeran's pencil is of "that form of pencil employing a permanent body or casing into which individual leads are inserted from time to time as may be desired". Among his aims: "to provide a magazine for extra leads which will be contained within the body of the pencil", to prevent the lead breaking when a new one is put in, and "to provide an indicator for determining the length of lead left within the pencil". The shell 10 is drawn to a taper at the front and may be thin gold or silver. Inside, a core or filler 12 "of some suitable fibrous material" has a bore 13 contracted at its front end 14, giving the lead a sliding fit along most of the bore and a clutching grip at the tip. A threaded sleeve 19 behind the filler receives the follower 22, "a stiff wire" with an enlarged head 23 of elongated cross-section, threaded on its outside to run in the sleeve. The magazine is a tube rolled flat with a slot that the follower passes through; turning the magazine turns the head and feeds the lead. A funnel-shaped depression in the magazine centres a newly dropped lead on the follower so it is not bent sideways and broken.

Take a mechanical pencil apart over a sheet of paper. Identify the parts that do Keeran's jobs: the grip on the lead at the tip, the feed, the reservoir for spare leads, and anything that shows how much lead is left. Draw each part. Then, on graph paper, write with 1 mm, 2 mm and 3 mm of lead showing, tilting the pencil to about 40°, and note the length at which the lead breaks. Compare with the notebook and estimate the lead's bending strength from your results.

**Charles R. Keeran** of Bloomington, Illinois, filed US 1,130,741 on 10 October 1913; it was granted on 9 March 1915. Screw-feed pencils with a permanent body were older; his patent covers the follower, magazine, filler and centring arrangement described above. **Honest limits.** Lead strength, writing force and thread pitch in the notebook are examples, not Keeran's figures.