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Mechanical Pencil Month – Day 8: The Interlock Pencil

Today’s post asks the question: What makes a mechanical pencil? Day three’s Nestler with its complex double-push action definitely qualifies; the simple cedar pencils of day two, clearly not. Going back to Hawkins and Mordan’s original 1822 patent for the “ever-pointed” pencil, it would seem that the basic requirements are a lead that can be advanced without the need for sharpening or cutting away and a holder that remains the same length throughout, regardless of how much lead is left.

However, within this definition there still remains enormous scope for debate. Automatic pencils such as the Faber-Castell TK-matic need practically no user involvement beyond replenishing the reservoir, thanks to a delicate spring-actuated sleeve that seamlessly advances the lead as it is used.

Faber-Castell TK-Matic L automatic mechanical pencil

At the other extreme, an eighteenth century porte-crayon does not require sharpening and its length stays the same, but it could hardly be considered a mechanical pencil.

Today’s pencil falls into this hinterland, with the added distinction of being the unwitting prototype for one of the most reviled pencils of all time. Sold as the “Interlock” pencil, this British-made curiosity is dateable by the patent number 510,649 helpfully printed on the lid of its rather tatty, but nevertheless remarkable survivor of a box.

Interlock pencil in its original box

The application was submitted in October 1938 by Herbert William James Jeffreys of Barking, Essex. I have been able to find out very little about the inventor’s life, except that he was proposed for membership of the Institution of Mechanical Engineers in 1906, at which time he was working as an “Engineer (Draughtsman) at W Warren [sic] and Company Limited, India Rubber Mills, Barking”. William Warne & Co was a large manufacturer of rubber engineering components, so presumably Jeffreys had experience in the design of plastic items for mass manufacture. Jeffreys’ nomination was clearly successful, as by 1910 he had become an associate member of the Institution.

Like many pencils of its day, the Interlock is made of marbled celluloid. However, unlike most others it is not machined from a single rod of the material, but instead built up of fifteen pieces, identical save for two that are bright red.

The idea is that as the lead is used up, the front section of the pencil can be unscrewed to expose a fresh length of lead. The removed section is then screwed into the back of the pencil, maintaining its overall effective length.

To indicate how much lead is left, the celluloid pieces into which the lead is fixed are coloured red so that, in the words of the patent specification, “its progress along the pencil is apparent”. As there are two of these stop pieces, it is possible to keep an unused length of lead in the pencil at all times, which also helps with its balance.

Jeffrey’s design was no short-lived gimmick, as after the second world war he took out an improved patent on the design, both in Britain and in France. This mainly covered a simplified way of retaining the lead, meaning that ordinary refills could be used instead of needing a special rubber retaining piece as the earlier models did.

Patent drawings for the Interlock pencil from Jeffrey's 1938 (left) and 1946 (right) British applications

Unfortunately, it seems that the pencil’s innovativeness failed to translate into to sales, and the Interlock is rarely seen today. The relative fragility of its celluloid parts and later perceptions of disposability presumably played into its fate.

For better or worse, its influence can still be felt on a product that has come to represent the excesses of our throw-away plastic culture, namely the Pop-a-Point pencil.

Pop-a-Point pencil from the Science Museum in London (cc)

Patented by Li Hung and Chao Fu Chin of Taiwan in 1966, the Pop-a-Point took the concept of removable stacking segments to its limit, with each plastic section containing just one short stub of lead. Unlike the Interlock, the removable sections did not form the body of the pen itself, but were housed within a conventional plastic barrel. Used sections were reinserted in the back, pushing forward a new unused length of lead. When the last section had been used, the pencil would simply be disposed of.

Whether the Pop-a-Point was ever meant to be a serious writing instrument is open to debate, but its legacy is clearly seen in the countless children’s crayons and pens based on the same concept, some of which almost return to the conceptual clarity and playfulness of the Interlock.

Children's coloured stacking pencil by Smiggle

What is lost in these derivatives is the idea of plastic as a modern wonder material that allowed the creation of new forms previously beyond the reach of the masses. The Interlock was conceived as anything but disposable, able to be refilled and used over and over again.

Sadly in today’s polarised world of industrial design, I find it easier to imagine a hypothetical Kickstarter campaign for an overpriced titanium Interlock clone, than I do any genuine attempt to bring longevity to everyday objects while simultaneously celebrating the potential of their materials.

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