 A solenoid is an electric output device that converts electrical energy into a linear mechanical force. They are utilized in a wide range of residential equipment, including doorbells, washing machines, and kitchen appliances. At its most basic level, a solenoid is an electromagnetic coil that uses magnetism, produced by the flow of current, to physically move the armature of a solenoid. The armature that is movable can be a rod or a metallic arm. A plunger solenoid contains a movable iron rod that is inserted into a coiled cylinder. The rod is connected to a plate with two contacts. With the aid of a spring, the contacts are normally open. When an electrical current passes through the coil, it generates a magnetic field that attracts the rod into the coiled chamber. The movement of the rod will move the contacts into a closed position. When the current is turned off, the magnetic force is removed and the tension in the spring forces the rod back out to its original resting position. This back and forth motion of the rod to control a circuit is used to operate a variety of mechanical devices. Solenoids can also function to control a secondary circuit using a metallic arm. When an electrical current is passed through a coil surrounding a metal core, again a magnetic field is created. In this assembly, the metal arm moves toward the metal core. Thus, the movable contact makes a connection with a fixed contact to close the switch and energize a secondary circuit. When the electrical current to the solenoid is turned off, the magnetic field disappears and the contact is broken as the spring returns to its resting position. When the mechanical armature of a solenoid controls the electrical contacts within another circuit, it is known as an electromechanical relay. Relays are primarily used when it's necessary to isolate a low power switch signal controller from a higher powered circuit or when several circuits must be controlled by one signal.