26 PHYSICAL MEASUREMENT OF X-RAYS. — BRONSON 



less easily absorbed the more lead they pass through. It 

 is for this reason that the jfigures in columns 3 and 4 have been 

 made to apply to rays which have already passed through 

 .026mm of lead or 2.0mm of aluminum respectively. 



Table III shows that the apparatus is well suited to 

 make comparisons of the hardness of X-rays. It is only 

 necessary to take two readings of the galvanometer, first 

 with one sheet of foil and then with two sheets. The ratio 

 of the readings may then be used as a measure of the hardness. 

 By obtaining foil of the right thickness, it would be possible 

 to arrange a simple scale which would correspond with any 

 of the various penetrometers now in use. 



Discussion of Results. 



The experiments above described show how a single 

 instrument may be very simply used to determine both 

 the quantity and quality of the Roentgen rays from any 

 tube, as well as the length of time of exposure needed to 

 produce radiograms of proper density. In each case the 

 ionizing action of the rays is made use of and the measure- 

 ments are all made with a galvanometer, which avoids the 

 uncertainty and difficulty of comparing the color or the 

 equality of illumination of two surfaces. 



The physical explanation would seem to be that the 

 magnitude of the effect produced by the rays on the photo- 

 graphic plate, the pastille and in ionizing the air depends 

 on the energy used up in each case, and that the relative 

 amounts absorbed in the three processes remain practically 

 constant for rays differing widely in their characteristics. 



The nature of the apparatus makes it comparatively 

 easy to calculate the number of ions produced per c.c. near 

 the photographic plate during the time of exposure or near 

 the pastille during the time of some definite change in color. 

 LetV = volumr- of testing vessel exposed to the ionizing 

 action of the rays. 



f = time of exposure in seconds of either plate or pastille. 



