An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it on top of which is a large fresnel lens that collimates the light. Above the box, typically on a long arm, is a mirror and lens that focusses and redirects the light forward instead of up.
The first overhead projector was used for police identification work. It used a cellophane roll over a 9-inch stage allowing facial characteristics to be rolled across the stage. The U.S. Army in 1945 was the first to use it in quantity for training as World War II wound down. It began to be widely used in schools and businesses in the late 1950s and early 1960s.
The primary reason for the gradual replacement of OHP's is the deeply ingrained use of computing technology in modern society and the inability of overheads to easily support the features that modern users demand. While an overhead can display static images fairly well, it performs poorly at displaying moving images. The LCD video display panels that were once used have fallen out of favor due to the limited resolution available and relatively dim, fuzzy image produced by the overhead.
What I like about using OHP's is it allows more direct interaction through live writing or movement on the transparency. A wind-up toy is any toy that is powered by a spring that is tightened by turning it. After release, the spring tries to get in its original, untightened position, and makes the small toy go as well.Wind-up toys date far back into history, into the fifteenth century. Karel Grod, a German inventor, created some of the first wind-up toys, including a metal fly and a mechanical eagle. Also, in 1509, Leonardo da Vinci created a wind-up lion as a greeting for Louis XII in
