Crookes tube

The Crookes tube opened a window into subatomic physics—though William Crookes didn't realize what he was seeing. His evacuated glass tubes, improved from earlier Geissler tubes by achieving much higher vacuums, produced mysterious glowing rays when electrical current passed through them. These cathode rays would reveal the electron, enable X-rays, and launch modern physics.

The adjacent possible had assembled from glassblowing and vacuum pump technology. Heinrich Geissler's tubes of the 1850s could produce partial vacuums and beautiful glowing discharges. But Crookes, using mercury diffusion pumps, achieved vacuums a million times lower. At these pressures, something strange happened: the glow retreated from the cathode, leaving a dark space, and an invisible ray traveled in straight lines to strike the opposite wall.

Crookes observed that these cathode rays cast shadows, could be deflected by magnets, and made the glass fluoresce where they struck. He believed he had discovered a fourth state of matter—radiant matter—streaming from the cathode. The tubes became objects of scientific fascination and drawing-room entertainment alike.

The cascade of discoveries came two decades later. In 1895, Wilhelm Roentgen noticed that a Crookes tube was causing a fluorescent screen across the room to glow. He had discovered X-rays—a form of radiation that passed through flesh but not bone. J.J. Thomson used Crookes tubes in 1897 to prove that cathode rays were actually streams of particles—electrons—smaller than atoms. The atom was not indivisible after all.

The tube's design—evacuated glass with electrodes at each end—became the template for vacuum tube electronics. Cathode ray tubes powered oscilloscopes, televisions, and early computers. The technology that began as a curiosity about glowing gases became foundational to twentieth-century electronics.

Crookes had built a window for looking at phenomena he couldn't explain. Others would understand what was passing through it.