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A solenoid coil is a wire tightly wound around a conductive core with a hollow center. When an electric current is passed through the coil, a magnetic field is created, thereby effectively forming an electromagnet. In most solenoid applications, this magnetic potential is used to perform some sort of work. In most cases, the energy provided by the electromagnetic energy is translated into simple linear motion to switch or actuate a variety of devices. Solenoid coils are generally modular units which can be removed and replaced if necessary, although sealed units are also fairly common.
Solenoids are among the simplest and most commonly used forms of linear actuation devices and rely entirely on the principles of electromagnetic energy for their operation. At the heart of any solenoid is the solenoid coil. This part of the solenoid consists of a tightly wound wire coil typically wound onto an insulated bobbin. The bobbin is fitted over a conductive core typically made of a ferrous metal that forms the body of the electromagnet. In most common configurations, the metal core is made with a hollow center designed to accommodate a spring loaded armature or plunger.
When an electric current is passed through the solenoid coil, an electromagnetic field is induced in the core. This pulls the armature against spring tension into the core's center, thus supplying the linear motion which actuates whichever device the solenoid is attached to. When the electrical supply ceases, the field collapses and the spring pulls the armature out of the core again. Not all solenoids work in a straight-line manner, however; rotary types represent other methods of harnessing the solenoid's electromagnetic potential. The principles of the solenoid coil and its role remain the same, though, no matter how the electromagnetic forces are applied.
Most solenoid coils operate in a closed environment, and as they generate a fair amount of heat, the chance of coils burning out is a constant possibility. For this reason, many solenoid coil equipped devices feature a modular construction that allows the coils to be replaced if they burn or become otherwise damaged. In some cases, the device may even be equipped with a coil of a different voltage rating if the need arises. Some solenoids do, however, feature coils and cores which are sealed into an enclosed unit. One of the better known sealed variants is the automobile starter solenoid used to engage the starter motor gear.