solenoid. An electromagnet with a movable iron core that can be pulled into the coil. When current flows in the coil, the resulting magnetic field pulls the core into its center. When no current is flowing in the coil, a spring moves the core out of the coil. Solenoids can be used to operate valves, switches, and mechanical locks, as well as many other types of devices.

solid. A material whose internal structure holds it tightly in its original shape and size. A solid differs from a liquid, whose structure causes it to hold only its size, not its shape, and a gas, that holds neither its size nor its shape.
solid conductor (electrical conductor). An electrical conductor made of a single piece of material, usually wire. Solid conductors are used where vibration is no problem, but stranded conductors are used where vibration is present.
solid fuel (rocket fuel). Rocket fuel made of a mixture of solid materials that, when ignited, combine fast enough to produce a great amount of heat. Solid fuel, which contains all the oxygen needed for the reaction, is molded into a shape that causes it to burn at the correct rate.
solidity (helicopter rotor system). The ratio of the total blade area to the disk area. The disk area is the area of the circle swept by the rotor as it rotates.
solid solution (metal heat treatment). A uniform combination of the alloying elements in a base metal. The alloying elements enter into the solid solution when the metal is heated in a furnace and held at a high temperature long enough for the metal to reach a uniform temperature throughout. When the metal is uniformly hot, it is removed from the furnace and immediately quenched to lock the alloying elements into the solid solution.
solid-state device. An electronic component that controls the flow of electrons without the use of any heated filaments, vacuum spaces, or moving parts. Semiconductor devices such as diodes, transistors, and silicon controlled rectifiers are examples of solid-state devices.