|MadSci Network: Physics|
This question is kind of like asking who discovered gravity. Electricity was discovered the first time somebody ran their fingers through their hair and zapped their friend on the ear, or stroked a cat's fur backwards to make it stand up, or saw lightning. One of the first experiments recorded was from ancient Greece when they found that rubbing something called Amber (fossilized tree sap) with a piece of fur caused pieces of straw to stick to it. Here is a good website explaining some of the best known experiments and early discoveries in electricity: experiments My favourite experiment is Galvani's experiment involving a dead frog. When Galvani touched a dead frog's leg with a knife the leg started twitching. Another scientist named Volta figured out that the knife and the metal plate on which the frog lay were forming an electrical "circuit" (a combination of things which cause electricity to flow) which made the frog's leg move. This brings us to your alarm system. The reason the alarm goes off when the trip wire is pulled is because it forms a circuit. When the circuit is open, the electricity basically stays in it's source (like a battery). When the trip wire is pulled, the circuit is closed, basically forming a tube through which the electricity runs to the bell (or buzzer, or whatever) and causes it to ring. Different bells ring for different reasons. Some use the electricity to generate a magnetic charge (magnetism and electricity are closely related) which pulls a metal bar over, hitting the bell, causing it to ring. Here are some other good website to check for information on electricity; Electrical Engineers group: ieee Lightning and Electricity Experiments (Theatre of Electricity): toe
Try the links in the MadSci Library for more information on Physics.