Rail gun
From HvWiki
A rail gun is a device that accelerates a projectile using electromagnetic forces. (Also abbreviated RG) Railgun theory is primarily based on the Lorentz force.
A rail gun consists of a pair of conductive rails each connected to a power source. A conductive armature/projectile then completes the electrical circuit between the rails resulting in current flowing up one rail through the armature/projectile and back down the other rail. When current flows through the rails and the armature/projectile a magnetic field known as the Lorentz force is created. This force is against both the rails and the armature/projectile, however since the rails are fixed the armature/projectile is forced down the length of the rails.
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Rail Gun Mathematics
Force
Where F is the force applied to the armature (Newtons), 
is the instantaneous current through the armature (Amperes), l is the length of the armature, and 
is the magnetic field vector through the armature.
Variations of Rail Guns
Solid Armature
The most common type for hobbyists, a conductive material such as graphite, aluminum, brass, or copper is used as the projectile/Armature(slug). Graphite is often favored for lower power designs as it does not damage the rails as much as other materials do and the fact that carbon that falls off rails can be relativly easily cleaned.
Plasma Armature
A piece of metal, such as aluminum foil, is put behind the slug to be fired. When current is applied, it turns from being a solid into the plasma matter phase, a high energy gas state.
Series Augmented Rail
A typical railgun consists of 1 turn or winding, a series augmented railgun adds turns in a series format, while still being powered by the same power supply as the main rails. A series augmented rail has the advantage of being able to tune the rails inductance to match that of the entire system, therefore reducing the damping effect and wasted energy. It also acts partly as a pulse-forming network with out wasting the energy that this type of network would consume.
Parallel Augmented Rail
A parallel augmented railgun is one method of lowering the current without reducing the electromagnetic force on the projectile. This is usually done by the adding additional rails in parallel with a separate high current, low voltage power supply, such as a battery bank, or dc arc welder type power supply.
Multi-Turn Rail/Armature
Power Source
Capacitors
Usually high voltage pulse capacitors are used. The high voltage is needed to overcome the slight resistance in the rails and other wires. Almost always capcaitors are stacked in series or parallel to get the desired voltage and current. Typically, capacitor banks are around 15 to 20 kilo Joules for a good railgun. Small banks(less than 1kJ) can be used for demonstrative purposes, but they will not signifigantly accelerate the projectile.
Compulsator
This is short for compensated pulsed alternator. It is a mechanical device which can store large amounts of kinetic energy in a spinning rotor.
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