Vacuum Tube

Vacuum Tube

No one man invented the modern vacuum tube or electron valve. It was the culmination of the work of many men. But we may list some of the most vital contributions.

Thomas Alva Edison, who invented the incandescent electric light, experimented with many substances in search of a suitable filament. In 1883, he discovered what is known as the "Edison effect." A third electrode was sealed by accident into one of his carbon lamps. When the lamp was lighted, a galvanometer showed a current in this third electrode, although it was isolated from the wore filament.

Sir William Crookes experimented with pot-bellied glass tubes from which the air had been exhausted until it was twenty million times thinner than the air we breathe. he sealed the poles of an electric circuit into opposite ends of a tube, and when a high voltage current was turned on he saw a strange light emanating from the cathode (negative pole). He gave the name "cathode rays" to this phenomenon, but, like Edison, he was unable to explain them.

Wilhelm Konrad Roentgen discovered in 1895 that it cathode rays in a vacuum tube are reflected from the anode, they produce rays of great penetrating force, capable of taking photographs right through solid objects.

In 1896, J.J Thomson, director of the Cavendish Laboratory at Cambridge University, England, announced his discovery of electrons. This not only laid the foundation for the subsequent rapid development of the modern theory of the atom, but also explained at once a number of puzzles. The cathode glow is caused by the free electrons forced out of the metal electrode by the force of the electrical current. Years later, the cathode glow was made the basis for an invention familiar to all of us the "neon sign." The brilliant red, green, blue or yellow lights in glass tubes, so extensively used for advertising signs, are caused by the friction of a stream of electrons passing through the neon or other gas with which the tubes are filled. The voltage of the current is stepped up to the point where the electrons are forced out of the cathode with sufficient velocity to reach the anode, and so fill the whole tube with light.