Middle Third Inductor

It marks the center section of an inductor drawing. An inductor is a wire coil that stores energy in a magnetic field. In the early 19th century, French physicist André-Marie Ampère used coils to study magnetism.

Why does the coil mark have three pieces?

It preserves one part of a text-drawing character set. The Unicode proposal to preserve older computer symbols maps it to Sharp MZ screen code E5 in the Japanese primary display set. The pieces were designed to sit beside line-drawing characters in text schematics.

Japan’s circuit-symbol standard changed in 1997, from wire-loop drawings to open half-ovals.