THE VACUUM — THERE 's SOMETHING IN IT — WHITNEY 199 



VACUUM TUBES 



The mere names of modern vacuum-tube applications of electrons 

 are legion. The kenotron is a vacuum tube which changes high- 

 voltage alternating current into direct current, and it has its counter- 

 part for low voltages in the battery-charging tungar rectifier and the 

 mercury rectifier. The various radiotrons, receivers, and amplifiers 

 of radio are also the most direct applications of the action of nega- 

 tive electrons in good vacua. But X-ray tubes must also be con- 

 sidered in this connection, because X rays are the result of the 

 " bump," if you will, of rapidly moving electrons in vacua against 

 the atoms of matter. 



Electrons in motion are also directed and controlled by electro- 

 magnetic, as well as by electrostatic, fields. Therefore the magnetron 

 and axiotix)n have to be included. Because the mere illumination 

 of such metals as potassium (like the high heating of other metals, 

 such as platinum and tungsten) causes them to emit electrons, the 

 photoelectric cell has to be included in our illustrations. 



The electrons within the vacuum, as in a radio detector tube, obey 

 the inconceivably feeble electrical impulses received by the antenna. 

 Conversely, the motion of electrons sets up impulses and waves in 

 space. In other words, the vacuum detector may be made a radio 

 wave generator. Such tubes are used in broadcasting stations. 



ETHEK WAVES 



The wave lengths of the magnetic waves produced by the changes 

 of motion of the electrons are very long in the case of radio (say 

 100 to 10,000 feet), millions of times shorter in ordinary light, and 

 millions of times shorter still in X rays, but they are all in the 

 same medium and all due to motions of electrons. 



SPECIAL EMISSIONS 



Finally, to give an impression of the distance such work has gone, 

 there should also be added the case of a one-atom-deep layer of 

 thorium on tungsten in a vacuum and its effect on electron emission. 



EXPERIMENTS 

 LAMPS 



The first great use of vacuum was in incandescent lamps. If such 

 a lamp is burned at much higher than its rated voltage, it lasts for 

 a few minutes only, but it gives a light perhaps five times as efficient 

 as we usually see. The lamp dies because the tungsten vaporizes or 

 melts. The vacuum is not at fault. It is because of these limita- 



