The simplest type of fluorescent lamp ballast, called a magnetic ballast, works like an inductor and generates a magnetic field when electric current is sent through a wire, according to HowStuffWorks. A basic inductor is composed of a coil of wire in a circuit, which can be wound around a piece of metal.
The field is amplified when the wire is positioned in concentric loops, states HowStuffWorks. This affects the objects around the loop as well as the loop itself. When the current in the loop is increased, the magnetic field is also increased, applying a voltage opposite the flow of current in the wire. This means that a coiled wire in a circuit opposes change in the current that flows through it. This principle is used to control the current in a fluorescent lamp.
HowStuffWorks notes that a ballast cannot stop changes in current. However, the alternating current that powers a fluorescent light constantly reverses itself, enabling the ballast to restrict increasing current in a specific direction for a short period. Magnetic ballasts are capable of modulating electrical current at a relatively low cycle rate, causing a conspicuous flicker. They can also vibrate at a low frequency, leading to an audible humming sound that people usually associate with fluorescent lamps.