Technical pen

A technical pen is a specialized instrument used by an engineer, architect, or drafter to make lines of constant width for architectural, engineering, or technical drawings. "Rapidograph" is a trademarked name for one type of technical pen. Technical pens use either a refillable ink reservoir (Isograph version) or a replaceable ink cartridge.

Early technical pens (ruling pens) consisted of a small pair of calipers, having one flat and one bowed leg holding ink between them. By adjusting the gap between the legs the width of the line drawn by the pen could be adjusted. Such pens, kept at a constant angle to the paper, were used for ruling lines, but not for cursive handwriting, nor for off-hand flourishes. The Graphos technical pen introduced in 1934 miniaturized the caliper principle and made the points easily interchangeable. The Sheaffer company produced an expensive drafting set which included such pens for use on linen prints. These sets were often presented to a draftsperson upon completing 'time served', which marked the end of the apprenticeship.

In the 1950s, fountain pens with cylindrical points became available, but they were complex instruments with tubes holding a tiny shaft. To release ink the shaft is depressed and a line of about the width of the exterior diameter of the tube can be drawn. Additionally, in later models, the tube had a small ledge that effectively narrowed its end, that—while maintaining the line thickness—made the tube thicker along most of its length and also protected ink from spilling while drawing along the edge of a rule, set-square, T-square or other template (the ink had no immediate contact with the template's edge). Some special, more expensive nibs were equipped with tubes made of tungsten or with their tips made of synthetic precious stones, to slow their wear on hard surfaces.

In the 1960s, the pen's design evolved to feature tubes of ink that were filled with a Pasteur pipette or from a narrow spout on a special bottle of ink. Such pens frequently came in sets of various sizes, and several pen points which were installed into the holders that also contained a filled fountain, which in turn would be screwed into a handle. The construction and number of parts varied depending on the company, and the parts were not cross-compatible in most cases. Some later designs (like the Staedtler MarsMatic700) had specially designed channels to allow better air flow in between the wall of the external grip and the point assembly. This made ink flow more reliable. The general drawback of this group of pens is that they have to be frequently and carefully cleaned to remove all ink from the tubing, otherwise it would set and could not be removed.

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