By assembling all of the aforementioned parts, the casing, the switch, the contacts, the light source, and the reflector, along with batteries and a lens, a useful portable light source is to be had. The durable "plastic" or metallic casing holds the batteries, contact strips or springs made of some sort of thin conductive metal; provides a place to mount the slide, rocker, or pushbutton on/off switch; acts as a base upon which to fasten the "head" of the flashlight which contains the light source (usually incandescent), the lens (usually plastic or glass), and the shiny aluminum reflector. All of the above components combine to form a small, light emitting, electrical circuit which may be turned off or on as needed.

Misell. "Electric Device," U.S. Patent 617-592, Jan. 10, 1899.
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Author Unknown. (2012, Feb. 6) Basic Electricity [Online]. Available: http://www.1728.org/project2.htm

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V. Enne. (2012, Feb. 6) How Does a Rocker Switch Work? [Online]. Available: http://www.ehow.com/how-does_5008389_rocker-switch-work.html

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Author Unknown. (2012, Feb. 6) Flashlights [Online]. Available: http://www.madehow.com/Volume-6/Flashlight.html#b