Popular Science Monthly 



401 



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. 



