SCIENCE— SUPPLEMENT 



ing #16 eleetrom currents, it is possible to deter- 

 mine accurately exacibly tow muoh tihorium is 

 present on the surface. An amount of thorium 

 corresponding to only 1/1000 of the surface cov- 

 ered with a layer one aitom deep is easily measur- 

 able in this way. It is possible to knock off a 

 thorium film by bombarding it with positive ions, 

 moving at high velocities, and in this way the 

 true najture of this bombardment can be deter- 

 miined. 



METHODS OP CONTROLLING ELECTRON CDBRENTS IN 

 HIGH VACUUM 



Most of the applieationis of high vacuum tubes 

 have depended upon the control of electron cur- 

 rents, as, for example, by the gmd in the three 

 electrode tube. The action of the grid is due to 

 the charge on the grid modifying the space charge 

 effect. This is the action (that is employed in 

 practically all tubes used to-day for radio trans- 

 mission and receiving. There are many other 

 methods, however, of controlling electron currenit^. 

 A very important method is thart used in the mag- 

 neton, where there are only two electrodes in the 

 evacuated space and the control is obtained by 

 moans of a magnetic field generated by an exter- 

 nal coil lof wire. A still simpler f o^rm of magneton 

 suitable particularly to very large power tubes, 

 consists of a very large filament in the axis of a 

 cyKndical anode wdtii very large sitraight fila- 

 ments. The magnetic field produced 'by the cur- 

 rent through the filament is enough to prevent 

 electrons flowing between cathode and anode. By 

 heating the filament with alternating current, the 

 current periodically falls to low value and at these 

 Ijimes current can flow to the anode. This gives 

 a pulsating or oscillating eurrenit, which can be 

 used for radio transmission. A 1,000 kilowatt 

 tube of this kind is in pirooess of development; 

 preliminary teats 'have been in every way satis- 

 factoiy. 



Ano'ther form of tube by whdeh electron cur- 

 rants can be controlled is the Dynatron. This 

 depends upon subjecitiing one of the three elec- 

 trodes in the tube to electron bombardment in 

 such a way as to cause eleotrons to be splashed 

 out of it, juist as waiter can be splashed out of a 

 cup by attempiting to fill it too ra,pidly from a 

 faucet. A tube of this kind acts like a real nega- 

 itjive reS'istanoe, and can be used for producing 

 electrical oscillations wth considerable efiioiency. 



One of the most important a,pplieations of elec- 

 tron discharges from hot cathodes is in -the 

 Coolidge X-ray tube which is now almost univer- 

 sally used as -a source of X-rays. These tubes 

 were first made abau,t 1913 and are gradually 



being improved in many respects. The 

 type of tube, suitable for use by dentists, is a 

 small tube weighing only a few ounces, and only 

 about three inches long. Because of the special 

 features of this itube, the emtire X-ray outfit, 

 including the transformer, lead screen, regulating 

 apparatus, etc., weighs only a few pounds and 

 takes up a space of only a small fraction of a 

 cubic foot. One very great advantage of this 

 new form of tube, besides its convenience, is its 

 absolute safety, even in the hands of inexperi- 

 enced operators, for there are no high voltages in 

 amy part of the apparatus which is accessible. 



EYE HOLDS SECRET OF TRAFFIC SAFETY 



Science Service 



A " LOOK-ONE- WAT " traf&c system as a pre- 

 vemtive of many of the frightful automobile acci- 

 dents of America 's deadly streets has been sug- 

 gested by Dr. Raymond Dodge, authority on 

 visual psychology and head of the psychologieal 

 section of the National Eesearch Council. Traffic 

 aigns, signals and routes should be determined ac- 

 cording to a itloToughgoing study of what the 

 human eje can see easiest. 



' ' In the present regulaton of traffic, both driver 

 and pedestrian never cover less than forty-five de- 

 grees of visual angle and may be required to look 

 over an angle of lOver two hxmdred degrees. Such 

 a range of vision is humanly impossible without 

 moving the head from side to side. This always 

 involves an interruption in the view of the part 

 of the sltreet from which trouble may come. One 

 of the greatest dangers in crossing the street 

 comes from vehicles tiat suddenly emerge from 

 a side street. 



" As a matter of safety there are grave doubts 

 as to whether the present regulations that limit 

 pedestrians to a narrow street crossing at exact 

 intersections of streets is the best practicable 

 solution. It may be more convenient for drivers, 

 but the exact intersection of streets is most dan- 

 gerous for pedestrians. Behind waiting cars is 

 safer than in front of them. Twenty feet from a 

 cross street would diminish the probability 'of 

 being surprised by turning ears. 



' ' Use of all parallel istreets as one-way streets 

 would be a great advantage. There is a sUgtit 

 but real difference between the sides of the street 

 for pedestrians. It can be demonstrated that the 

 left-hand sidewalk is safer. When the pedestrian 

 is on tie left sidewalk about to cross a street he 

 has to watch only automobiles on his left and to 

 the right of him, while a walker stepiping off tJhe 

 right hand sidewalk across a .street has to be alert 



