18 PHYSICAL MEASUREMENT OF X-RAYS. — BRONSON. 



Roentgen rays are commonly used for two distinct 

 purposes: 



(1) For diagnostic work by means of fluoroscope and 

 radiograph; 



(2) For their therapeutic action. 



In each case it is important to know both the quantity 

 or intensity and the quality or hardness of the rays furnished 

 by the tube. There are two general methods of measurement: 



(1) -The electric energy delivered to the bulb is measured 

 and it is assumed that all or a constant fraction of this leaves 

 the bulb as X-rays. A recent article by Dr. G. W. Holmes* 

 shows that experience would seem to justify this assumption. 

 This method does not distinguish directly between the quan- 

 tity and quality of the rays, but the applied potential is taken 

 as a measure of the hardness; 



(2) The quantity and quality of the rays themselves 

 may be directly measured; the quality by some form of 

 penetrometer, which involves the comparison of the intensity 

 of illumination of two surfaces; the quantity by the change 

 produced in the color of some substance, such as barium 

 platino-cyanide, in which case a color comparison is involved. 



In a few cases the ionization produced in air has been 

 used as a means of measuring the strength of the rays. Ex- 

 cept for experimental difficulties, this should be the ideal 

 method as practically all the evidence indicates that the 

 various effects of Roentgen rays are directly related to their 

 ionizing action. A simple direct reading instrument, making 

 use of this principle, has been recently described by Dr. B. 

 Szilardf. For some purposes this instrument should prove 

 very useful, but for others the length of time necessary to 

 get a reading would be objectionable. The instrument is 

 calibrated to give directly the total number of ions formed in 

 1 c.c. of the air exposed to X-rays during the time of an 

 exposure. 



*The American Journal of Roentc^enology, May, 1914. 

 t.\rchive3 of the Roentgen Ray, June, 1914. 



