J. M. Adams — Transmission of Rontgen Rays. 375 



Art. XXX YII. — The Transmission of Rontgen Rays through 

 Metallic Sheets y* by John Mead Adams. 



This subject presented itself in connection with an investiga- 

 tion of the energy of Rontgen rays as measured by a radiomi- 

 crometer. In that investigation the rays were allowed to fall 

 upon a thin sheet of platinum at one junction of a thermo- 

 electric circuit suspended in a magnetic field ; the heat devel- 

 oped by the absorption of the rays in the platinum was 

 measured by the deflections of the instrument. The necessity 

 of making correction for the incomplete absorption of the rays 

 in the platinum, together with the well known fact that the 

 character of Rontgen rays is changed by passage through sub- 

 stances, made it seem desirable to investigate the phenomena 

 of the transmission of the rays through metallic sheets. 



The general law of the absorption of the rays in a metal, 

 viz., that each successive equal increment of thickness is less 

 effective as an absorbing medium than the one preceding it, 

 was confirmed by experiments with the radiomicrometer ; and 

 curves showing the relation between the thickness of a metallic 

 sheet and its absorbing power were plotted and were found to 

 have the same general characteristics as similar curves which 

 other investigators had obtained by means of the fluoroscope, 

 the photographic plate, or the ionization electroscope. 



The dependence of the absorbing power of a given metallic 

 sheet upon the intensity of the rays incident upon it was 

 examined for sheets of silver, platinum, copper, tin, and 

 aluminium, and in every case it was found that the effective- 

 ness of a sheet as an absorbing medium is independent of the 

 intensity of the incident rays ; in other words, the deflection of 

 the radiomicrometer suffered an equal percentage reduction 

 upon the interposition of a metallic sheet in the path of the 

 rays, whether the latter were strong or weak. The rays were 

 weakened by moving the tube away from the rest of the 

 apparatus. 



It was found that the effect of the surfaces of metallic 

 sheets upon transmission is small, in the case of copper and of 

 aluminium, the only metals examined. To show this, a 

 laminated plate of the metal in question was prepared, equal in 

 total thickness to another solid plate of the metal. These two 

 plates were interposed in turn in the path of the rays, and pro- 

 duced equal reductions in the deflection of the radiomicrometer. 



The transmission of a beam of Rontgen rays through a 

 metallic sheet has generally been supposed to render the beam 



* Abstract of a paper published (April, 1907) in the Proceedings of the 

 American Academy of Arts and Sciences. 



