Solenoid

A solenoid is a coil of insulated wire that is many times longer than it is wide (in diameter). The magnetic field inside a current carrying solenoid can be shown to be proportional to the Amp turns per meter (length) and to the permeability of the core material m ( mo when filled with air).

The relationship has a remarkable consequence. The field strength inside a solenoid is independent of the diameter (provided the solenoid is long).

Solenoids were once widely used as switches - initiating mechanical movement when a current was turned on. The lever type turn indicators on cars were once operated by solenoids. An iron core was attracted into the solenoid when current flowed operating the arm directly. The simple system worked well until dust and grease jammed the mechanism. Solenoid operated levers have been replaced on all modern cars with flashing indicator lights.