What Makes an Electric Lamp-bulb Glow ?

Pelorus Guardian and Miners' Advocate., Volume 29, Issue 32, 27 April 1917, Page 2

What Makes an Electric Lamp-bulb Glow ?

When you heat iron in a forge it becomes either red hot or white hot, depending on how hot it is. It senc.s forth light. The hotter it is the more light it giver. Finely there comes a point where th; '.on melts away. The best light-giving material is that which will melt at the highest temperature. Carbon is a material which cannot be molted easily ; but it burns up in the open air long before it reaches the melting point. Edison conceived the idea of making a little thread of carbon, of placing that thread in a bulb, and of heating it by the electric current to the highest possible point. In order to prevent the carbon filament from burning up he pumped out all the air in the bulb. The result was that the thread of carbon was heated to the glowing point, so that it gave a very bright light. Tungsten is a metal which melts at the highest melting point. It ought to be the best light-producer, since it can be heated higher than any other metal without melting. The trouble is that tungsten is exceedingly brittle, so that a thread cannot easily be made of it. This difficulty was overcome about twelve years ago by making a paste of powdered tungsten and forming a thread of this paste. Later still a way was found of so treating the tungsten that it could be drawn into a hair-like thread a mile long if necessary. All modern electric incandescent lamps have such tungsten filaments. They consume very much less current than the older carbon filament lamps and give a much whiter light, simply because tungsten can be heated so very much before it melts.