Consider the humble paper clip whose utility highly depends on the properties of the metal developed by heat treatment. It may just be a thin piece of steel wire traditionally bent into a double-oval shape, but over the past century, no one has invented a better method of holding loose sheets of paper together. In effect, a paper clip consists of two metal surfaces that are pressed against one another by the elasticity of the metal wire from which the paper clip is made.
How it Works
As you distort the paper clip away from its original (equilibrium) shape by spreading the two surfaces apart and then releasing, it exerts pressure in the form of a force that tend to return the paper clip to its original shape by pushing the two surfaces together. Because the paper clip behaves like a spring, these restoring forces are proportional to the distance separating the two surfaces. When several sheets of paper are placed between the two surfaces, the restoring forces on the metal surfaces cause them to exert inward, compressing forces on the paper sheets. Because each sheet of paper does not accelerate, it is clear that the sheet experiences no net force. Instead, forces appear between each sheet of paper and its neighbors to oppose the compressing forces from the paper clip. The force between each sheet and its neighbor gives rise to friction between the sheets. The sheets cannot slide easily across one another because they will experience frictional forces whenever there is relative motion.
History of the Paper Clip
The common paper clip is a wonder of simplicity and function, so it may seem puzzling to learn that it is a relatively new invention. As early as the 13th century there was an interest in fastening paper, a common method being to put ribbon through parallel incisions in the upper left hand corner of pages. Later people started to wax the ribbons to make them stronger and harder to undo - this was the way people clipped papers together for the next six hundred years because straight pins, string and other materials used as fasteners punctured or damaged the papers they held.
While the paper clip seems like such an obvious solution, its success had to await the invention of steel wire, which was “elastic” enough to be stretched, bent and twisted. The design was perfected further by rounding the sharp points of the wire so they wouldn’t catch, scratch or tear the paper.
The wire paper clip was in production in Britain as early as the 1890’s. Although no known patent claims exist, The Gem Manufacturing Company is credited with designing the now familiar double-oval shape and has advertised it since 1898 (Fig. 1). In 1904, Cushman & Denison obtained a trademark for “Gem” used in connection with paper clips. The announcement of the trademark stated “Used since March 1, 1892.” Other notables include Johan Vaaler, a Norwegian inventor who patented a paper clip design in 1899 in Germany, since Norway had no patent laws at that time. It was never commercialized since better designs existed. A year later, in 1900, a U.S. patent for a paper clip, called the “Konaclip,” was awarded to Cornelius J. Brosnan of Springfield, Mass. Since then, literally zillions of paper clips have been designed and sold in every imaginable shape, size and color.
Although colorful plastic materials and new shapes have challenged the double-oval steel-wire paper clip over the years, none has proven superior. The traditional paper clip is the essence of form following function. After a century, it still works well.
Metal Fatigue and the Paper Clip
Just how many bends does it take to break a typical paper clip? The answer may surprise you. Our unofficial testing tells us between 11 and 30 bends, with 16 being the statistical average, we found if force is uniformly applied (caution: twisting the paperclip will cause it to fracture prematurely). Try it! The method involves bending each paper clip in 180-degree increments at a rate of one bend per second until it fractures. IH