Page:Popular Science Monthly Volume 88.djvu/429

 Popular Science Monthly

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��What Makes an Electric Lamp-Bulb Glow?

WHEN you heat iron in a forge it be- comes either red hot or white hot. depending on how hot it is. It sends forth light. The hot- ter it is the more light it gives. Final- ly there comes a point where the iron melts away.

The best Hght-giv- ing material is that which will melt at the highest tempera- ture. Carbon is a material which can- not be melted eas- ily ; but it burns up in the open air long before it reaches

the melting point. Edison conceived tlic 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 pre- vent the carbon filament from burning up he pumped out all the air in the bull). 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 heat- ed higher than any other metal without melting. The troul^le is that tungsten is exceedingly l)rittle. so that a thread cannot easily be made of it. This diffi- culty was overcome about twelve years ago by making a paste of powdered tung- sten and forming a thread of this paste. Later still a way was found of so treat- ing the tungsten that it could be drawn into a hair-like thread a mile long if nec- essary. All modern electric incandescent lamps ha\e such tungsten filaments. They consume very nnich less current than the older carbon-filament lamps and give a much whiter light, simply because tung- sten can be heated so very much before it melts.

Tllk^ Department of Agriculture as- serts that on the average farm a flock of one hundred to one hundred and fifty hens is more easily made profitable than a flock of one thousand.

��A Top That Never Stops Spinning,

ELECTRICITY has invaded the young boy's field of sportsmanship. The record spin in the game of whose- top-can-stay-up-longest has been shatter-

���A top which will keep on spinning forever — or until its battery wears out. It af- fords indeed "endless" amusement

ed SO badly that the cord-spun top, in comparison, really does not spin at all. Like most other things that electricity takes a hand in. the electrical top does not topple after a mere spin ; it whirls on for hours, according to the desire of its youthful operator. The top, in real- ity, is a miniature alectri*- motor turned on end. In place of the steel peg and the sidewalk, there is a steel shaft which revolves in a bearing, and instead of the wooden pear-shaped body, there is an iron armature wound with wire. At the top of the shaft varied colored disks are placed. When the current from a dry battery is turned on, the shaft revolves and the disks spin, giving a pleasing eft'ect in rainl)ow colors.

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