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A lemon is a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets.

In physics, the term solenoid refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space (where some experiment might be carried out).

In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid.

Electromechanical solenoids

Electromechanical solenoids consist of an electromagnetically inductive coil wound around a movable steel or iron slug (termed the armature). The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet. The armature is used to provide a mechanical force to some mechanism (such as controlling a pneumatic valve). Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very low reaction times.

The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil. The force applied to the armature will always move the armature in a direction that increases the coil's inductance.

The magnetic field inside a solenoid is given by:

${\displaystyle B=\mu _{0}NI,}$

where ${\displaystyle \mu _{0}=4\pi \times 10^{-7}}$ henries per metre, N is the number of turns per metre, and I is the current in amperes. See Electromagnet.

Electromechanical solenoids are commonly seen in electronic paintball markers, and dot matrix printers.

Rotary Voice Coil

This is a rotational version of a solenoid. Typically the fixed magnet is on the outside, and the coil part moves in an arc controlled by the current flow through the coils. Rotary voice coils are widely employed in devices such as disk drives.

Pneumatic solenoid valves

A pneumatic solenoid valve is a switch for routing air to any pneumatic device, usually an actuator of some kind. A solenoid consists of a balanced or easily moveable core, which channels the gas to the appropriate port, coupled to a small linear solenoid. The valve allows a small current applied to the solenoid to switch a large amount of high pressure gas, typically at around 100 psi (7 bar, 0.7 MPa, 0.7 MN/m²).

Pneumatic solenoids may have one, two, or three output ports, and the requisite number of vents. The valves are commonly used to control a piston or other linear actuator.

The pneumatic solenoid is akin to a transistor, allowing a relatively small signal to control a large device. It is also the interface between electronic controllers and pneumatic systems.

Hydraulic solenoid valves

Hydraulic solenoid valves are in general similar to pneumatic solenoid valves except that they control the flow of hydraulic fluid (oil), often at around 3000 psi (210 bar, 21 MPa, 21 MN/m²). Hydraulic machinery uses solenoids to control the flow of oil to rams or actuators to (for instance) bend sheets of titanium in aerospace manufacturing.

The basics of solenoid valves

Solenoid valves are the most frequently used control elements in fluidics. Their tasks are to shut off, release, dose, distribute or mix fluids. They are found in many application areas, for example: Fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design.

Besides the plunger-type actuator which is used most frequently, pivoted-armature actuators and rocker actuators are also used.

Solenoids in fiction

• The use of the word solenoid (particularly in science fiction) could be grouped in with other terms such as conduit, socket, firewall, capacitor, wormhole and laser to lend some kind of scientific/engineering credibility from a lay person's perspective.

• In the anime and manga series Neon Genesis Evangelion, one plot device was a "Super Solenoid Engine" (or S²), a limitless power source.

• In Steven Spielberg's 2005 War of the Worlds movie, the alien tripods disable all flow of electric current in a wide area, thus rendering vehicles useless; the problem is fixed by replacing the solenoid.

• In Gene Roddenberry's Andromeda TV-Series, an Anti-Proton Solenoid Valve is used to control the flow of the anti-protons which are used to power the ship and propel it through space. In the Episode "The Vault of The Heavens" (3.18), Engineer Harper orders Trance to shut it, filling the Anti Proton tanks to capacity, then expelling it quickly, thereby accelerating the ship much more than normal.

External links

• Solenoid Basics for Robotics
• Discussion of Solenoids at Hyperphysics
• Basics of Rotary Voice Coils

• Electromagnetic coils