TELEGRAPHY OF PHOTOGRAPHS BAKER. 



265 



way in which tlie\' may he utilized to solve a problem that has long 

 occupied many investigators, viz, the satisfactory measurement of 

 the beam of heterogeneous rays from an X-ray tube. Whenever a 

 new tube is used in radiographic w'ork, a different voltage, or different 

 interrupter or coil, the time of exposure for the photographic plate 

 has to be determined anew. The strength of the tube under any con- 

 ditions can, however, be determined by means of a simple piece of 

 apparatus which I have constructed, the working of wdiich I shall 

 now be able to show you. 



If the X rays fall on a fluorescent screen of barium platino-cyanide, 

 the screen absorbs them and emits yellowish-green visible rays; this 

 transformed energy is capable of affecting a very sensitive selenium 



^ £ 



IK. 



2k 



Curve SHowir^/Q errcr of fclblz 



I LLU M ll\/nT 10 N ON THE 



R£S/STANCE OF SELENIUM 



Resistance of unUiurmnafeii 

 celt 3fS,0D0 ohrrti 



Ci'tay Cell 



OZUbilpntU It tS 10 It, !l 32 3( 



4» 



2)istance tn lyie'hes of StLtNiOM CtiL fro 

 of Hlv-miytafion, x Z- 



Fig. ;!. 



^"01 



cell when placed in contact with the screen, the resistance becoming 

 less the greater the fluorescence. You will see here a selenium cell 

 of approximately 395,000 ohms resistance, over which is placed a 

 small fluorescent screen of the same size ; the cell is put in series with 

 a battery of 100 volts and a milliampere meter, the divisions of which 

 may be made to correspond to some arbitrary scale or to the time 

 necessary for the exposure of a given make of photographic plate. 



The dividing of the dial depends on two things: First, the char- 

 acteristic curve of the selenium cell connecting its resistance with the 

 strength of illumination, the linear distance of the source from the 

 cell being, in this case, the most convenient to employ. Second, this 

 characteristic curve must be modified to meet the case of illumina- 

 tion by the rays from the antikathode, which do not necessarily 



