The Production of X Rays of different Penetrative Values. 225 



that the potential difference, as measured by the alternative spark in 

 air, was much greater when the small cathode was in use than 

 with the larger cathode. 



The penetrative value of the X rays produced by any given tube 

 appears, therefore, to be dependent upon several conditions. 



1. The penetrative value is higher for a high vacuum than for a 

 low vacuum. 



2. It is higher when the electrical power applied is great than 

 when it is small. 



3. It is higher when the resistance of the tube is great than when 

 this resistance is reduced by magnetic means. 



4. It is higher when the distance between the cathode and anti- 

 cathode is small than when the distance is great. 



5. It is higher when the cathode itself is small than when it is 

 large. 



6. It is higher when, as a consequence of one or more of the above, 

 the potential difference between the cathode and the anode portion of 

 the tube, and consequently the electrical excitation of the cathode, is 

 great than when it is small. 



On the assumption that the cathode rays consist of negatively 

 charged molecules that are repelled from the similarly electrified 

 cathode with an initial velocity that depends upon the degree of 

 electrical excitation of the cathode, the above conditions are those 

 that would conduce to a high average velocity of the molecules at 

 the moment at which they strike upon the anti-cathode, and, at the 

 same time, to a high average difference of potential between the 

 travelling molecules and the anti-cathode at the moment of impact. 



At high exhaustions, not only is the electrical excitation more, and 

 the initial velocity of the molecules consequently greater, but, owing 

 to the smaller number of collisions with the other molecules of re- 

 sidual gas in the tube, the average velocity of the molecules and the 

 amount of their negative charge have suffered less diminution by 

 the time they reach the anti-cathode than in the case of lower 

 exhaustions. 



Similarly, when more electrical power is employed, the electrical 

 excitation and the initial velocity of the molecules is increased, while 

 the employment of a magnetic field reduces the resistance and the 

 electrical excitation, and consequently reduces the initial velocity and 

 negative charge of the molecules. 



Again, when the anti-cathode is near to the cathode, the moving 

 molecules having a less distance to travel before they reach the 

 anti-cathode, have by that time lost less of their initial velocity and 

 charge by collisions than when the anti-cathode and cathode are 

 further apart. 



And, lastly, not only does a small cathode become charged to a 



