Question

___ are devices that control fluid power within transmission lines

Answer 1

Valves  
Fluid power is the general term used for systems that use fluids to transfer power from one location to another. The fluid may be a liquid as in hydraulics, or a gas as in pneumatics. In either case, valves are used to control the transmission of power. The valves may be used as a safety mechanism (pressure release value), or as a control mechanism that is automatically, or manually activated.