Magnetic Field of a solenoid B = m0IN/L B = Magnetic field inside the solenoid  m0 = constant, permittivity of free space = 8.85x10-12 C2/Nm2 I = Current N = Number of turns of the wire L = Length of the wire

Solenoid Used in a Light Switch

Solenoid Used in a Doorbell

• Control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers
• Control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,
• Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),

• Isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,
• Logic functions. For example, the boolean AND function is realised by connecting normally open relay contacts in series, the OR function by connecting normally open contacts in parallel.