Solenoid
A solenoid is a 3 dimensional structure of wire. When this wire is wrapped around a metallic block in a coil and electricity is passed through it, it has some special magnetic properties. Electromagnetic induction makes it an electromagnet that can be switched on or off. The side in which the current appears to be passing clockwise, is the South Pole, and the side in which the current seems to be passing anticlockwise is the Northern Pole. The solenoid works just like a bar magnet, and therefore has many uses.
This principle is used to create valves, among other things. Where the solenoid operates an electric switch, it is a relay.
Pinball machines, vehicles and traditional door bells use solenoids.
Solenoid Media
Figure 1: An infinite solenoid with three arbitrary Ampèrian loops labelled a, b, and c. Integrating over path c demonstrates that the magnetic field inside the solenoid must be radially uniform.
The picture shows how Ampère's law can be applied to the solenoid
Examples of irregular solenoids (a) sparse solenoid, (b) varied pitch solenoid, (c) non-cylindrical solenoid
