ARTICLES 579 



mental one ; and one which is not provided for in the older 

 theories of electromagnetic radiation which have been hitherto 

 able to account for observed facts. Its application to the 

 present case would lead to the anticipation that the frequency 

 of the X-rays emitted as the direct consequence of the impact 

 of an electron would be equal to the product hv, where v is the 

 frequency and h the constant already referred to. The value 

 of this constant as determined from a comparison of the 

 original theories with experimental facts of radiation which 

 they were intended to explain was known to be nearly 

 6-5 x io" 27 . 



There is at Harvard University a set of no less than 20,000 

 small storage cells. With this battery a very steady and 

 measurable voltage can be put on to an X-ray bulb. As the 

 voltage increases, so does the energy of the electron which 

 excites the X-ray, according to the rule 



Ve = Imv 2 



where e and m are the charge and mass of the electron and v 

 its velocity. It has been found (Webster and Duane) that 

 no X-rays of frequency v can be generated in the tube until the 

 applied potential has been raised to the value V where 



Ve = hv, 



that is to say, until the energy of the electron is equal to the 

 anticipated amount (see fig. 4). Whatever the anticathode 

 in the bulb there is always a general emission of X-rays of all 

 frequencies ranging from the critical value downwards to 

 smaller values. The critical value is very sharply defined. 

 In fact, if it is assumed, as it is natural to do, that this inter- 

 pretation of the results is correct, we have here a very excel- 

 lent method of determining h. The following table shows 

 the result obtained (see Blake and Duane, Physical Review, 

 December 191 7, p. 637). Some previous results are also given. 



By Means of X-rays 



Duane and Hunt, Phys. Review, August 191 5 . . 6*51 x io - * 7 



Hull, Phys. Review, January 1916 .... 6*59x10"" 



Webster, Phys. Review, June 1916 .... 6*53xio -,T 



Webster and Clark, Proc. Nat. Ac. Set. p. 181, 1917 . 6*53 x icr" 



Blake and Duane, Phys. Review, December 1917 . 6*555 x io - " 



