circuit breaker (electrical circuit component). A device which opens an electrical circuit when an excess of current flows. A circuit breaker may be reset to restore the circuit after the fault causing the excessive current has been corrected.
Circuit breakers may operate either by heat or a magnetic field, and some have an exposed control so they may be used as a switch as well as a circuit protector.

circuit diagram. A drawing which uses conventional symbols to show the way the components in an electrical system or fluid power system are connected. Circuit diagrams do not show the physical relationship of the components, but the flow of electrons or fluid can be traced by following the circuit diagram.
circuit protection (electrical circuit). Provision in an electrical circuit to prevent the flow of an excessive amount of current. Circuit breakers, fuses, and current limiters open the circuit when excessive current flows.
circular inch. A measurement of area of circular objects. One circular inch is the area of a circle whose diameter is one inch. One circular inch is equal to 0.7854 square inch.
circular magnetization. A method of magnetizing a part for magnetic particle inspection. Current is passed through the part, and the lines of magnetic flux surround it. Circular magnetization makes it possible to detect faults that extend lengthwise through the part.
circular mil. A measurement of area used for round electrical conductors. A circular mil is the area of a circle whose diameter is one mil, or 0.001 inch. One circular mil is equal to 0.7854 square mil.

circular motion. Motion of an object along a curved path in which the object stays a constant distance from a given point, called the center of motion.
circular saw. A powered saw that uses a thin, circular blade driven by an electric motor. In a table saw, the blade is mounted on an arbor, and only a portion of the blade sticks up above the surface of the table. In a radial-arm saw, the blade is mounted on the shaft of an electric motor which is suspended on an arm above the work. The saw is pulled across the work. A circular saw blade may also be mounted on a hand-held electric motor and used as a hand saw.