<P> A rotary voice coil 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 . The working part of a moving coil meter is also a type of rotary voice coil that pivots around the pointer axis, a hairspring is usually used to provide a weak nearly linear restoring force . </P> <P> A pneumatic solenoid valve is a switch for routing air to any pneumatic device, usually an actuator, allowing a relatively small signal to control a large device . It is also the interface between electronic controllers and pneumatic systems . </P> <P> 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 . Solenoid - controlled valves are often used in irrigation systems, where a relatively weak solenoid opens and closes a small pilot valve, which in turn activates the main valve by applying fluid pressure to a piston or diaphragm that is mechanically coupled to the main valve . Solenoids are also in everyday household items such as washing machines to control the flow and amount of water into the drum . </P> <P> Transmission solenoids control fluid flow through an automatic transmission and are typically installed in the transmission valve body . </P>

What is the magnetic field of a solenoid