PHYSICAL MEASUREMENT OF X-RAYS. — BRONSON. 23 



Table I. 



Plate 

 No. 



Milliam- 

 meter. 



Spark Gap. 



Lead. 



Gal. Defl. 



Exposure. 



91 



1.1 



unknown 



.104mm. 



23 



9 sec. 



95 



4.1 



unknown 



.208mm. 



16 



12.5 sec. 



96 



3.1 



unknown 



.208mm. 



29 



7 sec. 



10 



0.38 



18cm 



.104mm. 



7 



29 sec. 



10 



9.38 



18cm 



.156mm. 



5.5 



36 sec. 



107 



0.50 



9cm 



.104mm. 



5 



40 sec. 



A comparison of the currents through the tube with the 

 galvanometer deflections, in the cases of 95 and 96 and of 

 105 and 107, shows how unsafe it is to judge the intensity of 

 the X-rays by the milliammeter alone. In both cases the 

 current through the tube increased, but the galvanometer 

 reading decreased, and the radiographs showed that the 

 desired results were obtained by increasing the time of ex- 

 posure, as w^as actually done. We must conclude then that 

 the relative action of Roentgen rays on a photographic 

 plate and in ionizing the air near the plate remains practi- 

 cally constant, however the rays themselves may be altered. 



In order to compare the absorbing power of various 

 parts of the body wnth various thicknesses of lead foil, radio- 

 graphs of the knee, the thigh and the chest were taken with 

 lead foil of varying thicknesses on the same plates. The 

 results were rather unsatisfactory, because somewhat dif- 

 ferent results were obtained with rays of different hardness, 

 and in practice it would be better to use aluminum sheet 

 in place of lead foil for this purpose. However, the negatives 

 showed about the same density for the following when the 

 penetration was 8 or 9 Benoist: 



Knee joint and 8 thicknesses of foil each .026mm. 



Thigh bone and 10 



Flesh of thigh and 8 " " 



Ribs and 10 or 12 " 



