circuitus

Circuitus comes from circumire — to go around. Circum means around, and ire means to go. A circuit is a path that returns to its starting point. Roman legal circuits — judges traveling a set route through provincial towns — were circuits in the literal sense: circular journeys. English circuit courts preserve this usage. The judge rides the circuit. The circuit is the route.

The electrical meaning appeared in the nineteenth century. An electrical circuit is a closed path through which current flows — from the power source, through the load, and back to the source. If the path is broken, the circuit is open, and current stops. The metaphor is exact: electricity goes around. If it cannot complete the circle, it does not go at all. The Latin going-around became the English wiring diagram.

The integrated circuit — a miniaturized electronic circuit on a semiconductor chip — was invented independently by Jack Kilby (Texas Instruments, 1958) and Robert Noyce (Fairchild Semiconductor, 1959). Kilby's first chip was a single transistor on a germanium wafer. Modern chips contain billions of transistors. The 'circuit' on the chip is the same Latin circle, only smaller. Much smaller.

The word works in three registers simultaneously. A circuit court is a traveling court. An electrical circuit is a path for current. An integrated circuit is a chip. In racing, a circuit is a track. In fitness, circuit training is a sequence of exercises performed in rotation. All of them go around. All of them return to where they started.