Crookes tube

The Crookes tube is an electrical discharge tube. It was invented by the British scientist William Crookes who gave it its name.

The discovery of the Crookes tube

After learning to control electricity, Germans observed electrical discharges in Geissler tubes (today known as plasma), and these discharges produced light. Magnetic fields were observed to deflect these discharges. The British physicist and chemist William Crookes discovered that when the pressure was lowered the cathode appeared to emit light rays, called cathode rays To make his observations Crookes had coated the walls of the tube with a phosphorescent material. The first cathode ray tubes were thus called Crookes tubes.

Diagram of a Crookes tube. © Chris Burks Chetvorno,
Public domain

The Crookes tube principle

When the cathode is heated it emits rays that are propagated to the anode. The inner walls of the tube are coated with a phosphorescent material so they emit light.

These are the rays that are responsible for luminescence and which are propagated in straight lines. Light is emitted when the rays strike the phosphorescent coating.

A 1960 Motorola Model 23SF3 cathode ray tube television set. © Public Domain

In a cathode ray tube the electrons are focused either magnetically or electrostatically by a grid (which controls the density of the beam) so as to obtain a narrow beam such as in a TV tube. The whole device is called an "electron gun". The beam leaving the electron gun is deflected either magnetically (as in the TV tube), or electrostatically (in oscilloscopes). This beam arrives at the anode which is covered in phosphorescent material which is often rare earth-based. When the electrons strike this surface, light is emitted in the form of a spot.