EVOLUTION OF THE VALVE

Evening Star, Issue 23881, 10 May 1941, Page 4

EVOLUTION OF THE VALVE

DE FOREST'S EXPERIMENTS In an article in the ‘ Radio News,’ Dr Lee de Forest recently set down a record of his experiments resulting in the development of the valve, and thus contributing possibly the most important advance in the growth from wireless telegraphy to telephony. The evolution of the radio, or ‘ audion tube, as it was known,” he writes, is a very interesting story, and one that is not at all well known. Most radio men assume that it was accomplished through a fling of inspiration. But this is not true. It came about through hard work. “ In 1900 I entered into the development of wireless telegraphy, and, knowing at that time what Marconi had been using, there were many unnecessary complications. I did not have a 'clue, but 1 went to the library in Chicago at night _ and finally came across an observation which a German physicist had recorded, which gave me an idea of what could be developed into a wireless detector. I worked in my room at night and spent my days in the Western Electric laboratory. In my room was a little spark coil with which I generated my electrical waves. One night, when 1 made this coil apark, I noticed the light of the Welsbach gas burner on the wall dimmed very perceptibly. It occurred to me that I had actually made a discovery, that the electric waves were acting upon the incandescent gases surrounding the gas mantle. “ In 1903 a chance to get into laboratory work and investigate the gas detector, as I called it. 1 used the Bunsen in these experiments, and proved that heated gases were actually responsive to electric waves. By 1905 I had advanced to the point where I was using a carbon filament to heat the attenuated gases in a glass tube. In connection with this bulb, I used, as I had always used in my gas flame experiments, a telephone receiver with a B battery connected between the plate and the filament in the bulb. The device was not a rectifier, but a genuine relay detector, whereby the electric waves produced marked changes in the battery current which \Vas flowing through the tube. In 1906 I removed the antenna connection from the plate electrode and connected it to a simple piece of tin foil wrapped around the cylindrical tube. This proved to be a great improvement on my preceding arrangement. I next placed this controlled electrode within the tube in the form of another plate on the opposite side of the filament from the first plate. This third electrode within the tube was a marked improvement, and I decided that I could still further improve the device if I worked it between the filament and the anode electrode. “At this time I had in mind a telephone repeater or relay, and took out a broad patent on the three-elec-trode tube thus used. “ In addition to the filament battery and the plate or B battery, I used a C battery in a series with the controlled electrode. My patent was that used this battery to bias 1 negatively * the controlled electrode, but I did not claim this arrangement in my patent. As a result of. this omission on the part of the patent attorney, Mr Lowenstein later secured a patent on the negative grid bias which for years was a controlling patent in radio litigation. The negatively-charged controlled electrode was of much more value when the audion was used as a telephone relay than as a wireless deFrom my earliest experiments I continued to use a blocking condenser in a series within the controlled electrode.” (To be concluded.)