THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] , 



SEPTEMBER 1917. 



XVII. Penetrating Power of the X Radiation from a Coolidge 

 Tube. By Sir E. Rutherford, F.R.S., Professor of 

 Physics, University of Manchester* . 



THE present paper contains an account of some expe- 

 riments made to determine the maximum penetrating- 

 power of the X rays excited by high voltages in a Coolidge 

 tube, using lead as the absorbing material. Owing to the 

 lack of time at my disposal, the experiments, made a year 

 ago, are incomplete ; but they may prove of interest in indi- 

 cating the penetrating power of the X radiation that can be 

 obtained from this source under practicable conditions, and 

 in throwing light indirectly on the probable frequency of the 

 very penetrating gamma radiation from radioactive bodies. 



In these experiments, the absorption of the X radiation 

 by lead has been examined over a very much wider range 

 of intensity and of thickness of absorber than in the original 

 experiments of Rutherford, Barnes, and Richardson f . 



To excite the radiation , a large induction-coil of 20-inch spark 

 was used, actuated by a mercury motor-break in an atmo- 

 sphere of coal-gas. The heating current through the tungsten 

 spiral was adjusted to give a radiation of maximum intensity 

 at the voltage required, which was fixed by an alternative 

 spark-gap between points. The radiation was found to be 

 most constant when a fairly rapid stream of sparks passed 

 between the points during the measurements. The well- 

 insulated Coolidge tube was placed inside a large lead box, 

 and the X rays, issuing through a rectangular opening in 



* Communicated by Ike Author. 



t R.. B., and R, Phil. Mag. xxx. p. 339 (1915). 



Phil. Mag. S. 6. Vol. 34. No. 201. Sept. 1917. M 



