Maximum Frequency of X Ray s from a Coolidge Tube. 351 



added, and the current measured with the capacity removed.. 

 An exactly similar process was carried out at 145,000 and 

 175,000 volts, and the variation of the ratios of the two 

 currents allowed o£ an accurate measure o£ the small 

 change in absorption coefficient in the three cases. 



Variation of Absorption of End Radiation with Voltage. 



The curve showing the values of fi in aluminium for the 

 end radiation at different voltages is shown graphically in 

 fig. 4, where the abscissae represent volts, and the ordinates 

 log p. The experimental points are indicated in the figure, 









Fig. 4 























































Absorption of end radiat 

 at different voltages 



on 





















' 







^ 



*/ 









°£!2r-" 



Ct "^So — "" 













t*i 

























90 70 



Thousands of Volts. 



and from the smoothed curve the values of //, for different 

 voltages are given in the following table. Taking X = ^ 2 / 5 , 

 the values of the wave-length \ are calculated, the value of 

 k being deduced from the data of Bragg previously quoted, 

 viz. fM=5-6 (cm.)" 1 for X = 5'0 x 10" 9 cm. 



It will be seen that the value of /j, decreases rapidly at 

 first with voltage, but decreases slowly after 100,000 volts, 

 and very slowly after 125,000 volts. There is no measur- 

 able change in /x between 145,000 and 175,000 volts. 



During recent years, experimental evidence of various 

 kinds has indicated that the energy of radiation is emitted in 

 definite quanta, expressed in the simplest case by Planck's 

 relation E = hv, when E is the energy of the ray and v 

 its frequency, and h Planck's fundamental constant, which 



