Double Header: 40 Inch Lens and The Edison Effect

Last week this photo was posted by Pierre Seguin and comes from a 1947 edition of Popular Mechanics. It shows a 1945 RCA Orthicon camera with a 40 inch lens, described as the longest ever used at the time. It also mentions that television works only with the help of the Edison Effect…a discovery by Edison that led to the invention of the vacuum tube by James Fleming.

The “Edison effect” was the name given to a phenomenon that Edison observed in 1875 and refined later, in 1883, while he was trying to improve his new incandescent lamp. The effect was that, in a vacuum, electrons flow from a heated element — like an incandescent lamp filament — to a cooler metal plate. Edison saw no special value in the effect, but he patented it anyway. Edison patented everything in sight. Today we call the effect by the more descriptive term, “thermionic emission.”

Now the Edison effect has an interesting feature. The electrons can flow only one way — from the hot element to the cool plate, but never the other way — just like the water flow through a check valve. Today we call devices that let electricity flow only one way, diodes.

In 1904, the Edison effect was finally put to use, but not in a light bulb. Radio was in its infancy, and the British physicist John Fleming was working for the British “Wireless Telegraphy” Company. He faced the problem of converting a weak alternating current into a direct current that could actuate a meter or a telephone receiver. Fortunately, Fleming had previously consulted for the Edison & Swan Electric Light Company of London. The connection suddenly clicked in his mind, and he later wrote,

“To my delight I … found that we had, in this peculiar kind of electric lamp, a solution!” Fleming realized that an Edison-effect lamp would convert alternating current to a direct current because it let the electricity flow only one way. Fleming, in other words, invented the first vacuum tube. Of course, most vacuum tubes have been replaced with solid-state transistors today; but they haven’t vanished entirely. They still survive, in modified forms, in things like television picture tubes and X-ray sources.

Source