SiWir. CONTEMl'OR.IRy .tDl'/LXCES IN I'liySICS-ll 



173 



lio\vi'\'cr, they have not so iiuicli niern\- as tlio primary or ilu- tincc- 

 deflectetl electrons, they am be kept a\va>- from the collector by lower- 

 ing its potential to a value such that only such electrons as have, 

 s;iy, \W(, of the energy of the primary electrons can reach it. Thus 

 the filament may be at potential zero, the target at otH) voli^; if the 

 collector is also at 500 volts, the (listrii)uti()n-in-anglc fimciion of the 



Fig. 12 



electrons it receives has nothing in common with the function F 

 characterizing the once-deflected electrons; but when the collector 

 is loweretl to .50 volts, the distribution-in-anglc function which it 

 records assumes a new and characteristic form. 



Some of these angular distributions are shown in Fig. 12 (for mag- 

 nesium) and Fig. 13 (for platinum). The latter curves were obtained 

 first, with a platinum target; then the target was overlaid with a 

 thin film of magnesium, formed by sublimation without opening 

 or altering the tube, and the sharply-contrasted curves of Fig. 12 

 replaced the others. The distribution-in-angle of the engineering 

 electrons is plotted, naturally, in polar coordinates; the direction 

 ^ = 0°, i.e., the direction of motion of the primary electrons, is indi- 



