58o 



SCIENCE PROGRESS 



In converse experiments, when e.g. light rays of a given 

 frequency cause the emission of electrons from certain sub 

 stances (photoelectric effect), a similar relation is known to 

 hold. The energy of the fastest electron which is emitted 

 from the surface of the substance is related to the frequency 



30 



Kilo volts 



Fig. 4. 



The figure is taken from a paper by Duane and Hunt, Phys. Rev. August 19 15, p. 166. The crystal 

 and the ionisation chamber of the X-ray spectrometer are set to catch rays of a certain narrow range of 

 frequency v, and the volts on the X-ray tube are gradually increased. Nothing appears until Ve = hv. 

 This limit being passed the rays come in with a rush, and their amount, as measured in the ionisation 

 chamber of the Spectrometer, increases very rapidly as the voltage is increased still further. The anticathode 

 of the X-ray bulb was of tungsten. None of the narrow ranges chosen for examination, and each represented 

 by one of the curves to the figure, contained a characteristic line of the tungsten spectrum. 



by the law given above. In this way have been determined 

 the results given below, quoted from the same paper. 



Determinations of h by Means of the Photoelectric Effect 



Millikan, Proc. Nat. Ac. Set. April 191 7 



Kadesch and Hennings, Phys. Rev. September 1916 



Sabine, Phys. Rev. March 191 7 . 



. 6*57 xicr" 

 . 643x10-" 

 (6-58- 671) xio- ,? 



Webster, Phys. Rev. June 191 6, has measured the electro- 

 motive force that must be applied to the X-ray tube to pro- 

 duce the characteristic radiations of the target as distinguished 



