How Does a Tape Measure Work?
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The Basics
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There are two basic types of tape measures. Tapes used for tailoring or sewing are usually made of fabric, plastic or fiberglass, and simply consist of a flexible tape marked with dimensions. It's common to find metric units on one side and imperial units (US and UK) on the other. Less common are flexible tapes like these that use a spring to roll and unroll from a case, which work similarly to tape measures from the 19th century.
Invention of the Modern Tape Measure
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Standard spring tape measure
What we more often think of as a tape measure is the spring tape measure used in construction and many other professions, as well as being an essential item for even the most modest toolboxes. While inferior spring tape measures existed by the 1860s, the modern spring tape is based on the design of Alvin J. Fellows, who patented his invention in 1868 (see his original patent and diagrams at http://www.todayinsci.com/F/Fellows_Alvin/FellowsTapeMeasurePatent.htm). His design used a fabric tape, but was the first to add a working button to hold the tape at a desired length.
Inner Workings
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Today's standard spring tape measures use a tightly coiled flat metal spring that is attached to the end of the measuring tape, which can be 25 or 50 feet or more. The other end of the measuring tape has a flat metal hook that allows you to hook on to one end of what you are measuring, and turns measuring something longer than your arm from a two- to a one-person job. The metal measuring tape is designed to curve in the middle, because this keeps the tape measure straight and allows even long lengths of tape to stay straight without support.
When the tape is pulled from the casing, the spring inside it starts to unwind, storing energy in the metal spring itself that will be used to wind the tape up again. Standard tape measures have a hold switch that grabs and holds the tape at a certain point, preventing the spring inside from rewinding itself. When the hold is released, the spring pulls the tape back into the case. This can happen very quickly with surprising speed and force, so always be careful when rewinding your tape measure.
Variations
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Hook on the end of a standard metal measuring tape
Several recent variations on the standard spring tape measure design have been introduced to improve usability or for specific applications. The Magna Measure (http://www.popularmechanics.com/home_journal/tools/1274146.html) is a simple addition to a standard tape for metalworkers and others working with metal surfaces. The Sabrr tape measure (http://www.sabrr.com/features.htm) combines a utility knife, pencil, and tape measure into a single ergonomically designed unit. But even this update has the same functional heart as the first spring tape measure.
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