An
arc lamp or
arc light is the general term for a class of lamps that produce light by an
electric arc (also called a voltaic arc). The lamp consists of two
electrodes, typically made of
tungsten, which are separated by a gas. The type of lamp is often named by the gas contained in the bulb; including
neon,
argon,
xenon,
krypton,
sodium,
metal halide, and
mercury. The common
fluorescent lamp is actually a low-pressure mercury arc lamp.
The electric arc in an arc lamp consists of gas which is initially ionized by a high voltage and therefore becomes electrically conductive. To start an arc lamp, a very high voltage is pulsed across the lamp to "ignite" or "strike" the arc across the gas. This requires an electrical circuit with an igniter and a ballast.
The ballast is wired in series with the lamp and performs two functions, when the power is first switched on, the igniter/starter (which is wired in parallel across the lamp) sets up a small current through the ballast and starter. This creates a small magnetic field within the ballast windings. A moment later the starter interrupts the current flow from the ballast, the ballast has a high inductance therefore it tries to maintain the current flow (The ballast opposes any change in current through it) it cannot as there is no longer a 'circuit'.
As a result, a high voltage appears across the ballast momentarily - to which the lamp is connected, therefore the lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp. The circuit will repeat this action until the lamp is ionized enough to sustain the arc.
