Vacuum Tube, Invented The Valve

Vacuum Tube, Invented The Valve

The work of Thomson spurred a new era of experimentation. Sir John A. Fleming put the previously useless "Ddison effect" to work in 1904-5. He built the Fleming valve, a tube with two electrodes instead of one. This so-called "diode" was and still is the best of all "rectifiers." A rectifier is a device for changing alternating current to direct current. Electrical power can be transmitted over long wire lines only by alternating current, but for some purposes direct current is preferable or essential. If alternating current is fed into the cathode of a vacuum tube, the electrons forced out of it can flow only one way toward the anode.

An experiment of epochal importance was made in 1906 by Dr. Lee Forest. He inserted a grid a screen of fine wire, between the electrodes of a Fleming tube. When a small negative potential was impressed upon the grid, its repellent effect on the stream of electrons from the cathode greatly reduced the number that got by to the anode. Conversely, a positive potential in the grid greatly accelerated the flow from cathode to anode.

The first use of this so-called "valve" was to make a galvanometer of great sensitivity. In other words, when the grid is connected to a circuit through which are passing currents much too weak to be detectable by former devices, the effect on the electron-stream in the vacuum tube is so great as to permit very accurate measurement.

Then it was realized that the valve could be used in another way. A weak positive potential in the grid can be amplified to relatively huge proportions by its effect on the cathode-anode flow. Dr. Irving Langmuir developed the De Forest tube for this purpose, rearranging the parts and incidentally inventing a mercury vacuum pump far superior to its predecessors. This development of the extremely sensitive amplifying valve made possible at once the transmission of voice by Hertzian waves that is radio. All principles of radio had been worked out, but the practical device waited upon a "detector" far more sensitive than was used in wireless telegraphy.