Some machines are so simple and deeply ingrained into everyday life that we seldom pay them much attention. Take the zipper, for example.
The zipper is a relatively modern invention, yet it uses two ancient tools: the wedge and hook. A wedge is an object with a slanted surface that exerts force on other objects in order to move them up or to the side. A hook, as everyone knows, is simply a curved object used to fasten on to another object. A hook normally fastens by latching onto a loop or hollow on a piece of material.
If you take a close look at a zipper, you’ll see that it consists of two tracks, each containing teeth spaced evenly apart. The teeth are hooks, the spaces in between the teeth hollows. A zipper works because the tracks are designed so that the teeth on one track line up opposite the hollows on the other track. When the tracks are brought together, each tooth fits snugly into its corresponding hollow, forming a tight bond.
But how are the tracks brought together in the first place? That’s where the wedge comes in. Actually, a zipper uses a series of wedges to guide the tracks into place. When you zip up your jacket you use a device called a slide, which consists of a handle used to pull a triangular piece of metal up and down the zipper tracks. When you pull the slide up, the tracks enter on an angle and are forced together by a wedge system inside the slide. When you pull the slide down, the wedges push into the interlocking teeth and force them apart. Simple but effective.