PAPERS OX CHEMISTRY AND PHYSICS 233 



meter and the battery circuits were closed continuously 

 so that trouble with thermoelectric effects was eliminated. 

 Under ordinary ciroimistances the galvanometer deflec- 

 tion exhibited a steady shift resulting from a gradual 

 change in the X-resistance caused by its change in tem- 

 perature. By readmg the deflection at intem-als of, for 

 instance, fifteen seconds with the X-ray beam alternately 

 on and off, the effect of the X-rays themselves could be 

 isolated. The results may well be shown graphically, 

 plotting times as abscissa and deflections as ordinate. 

 "When temperature changes are occurring, the points 

 will lie on a smooth cur\'e ha^dng a iDOsitive slope. If a re- 

 sistance change is produced by the radiation, alternate 

 points will be displaced vertically from the cunx by a 

 constant amount. The amount of this displacement gives 

 a measure of the resistance change. 



Some difficulty was experienced at first with leakage 

 from the high tension circuit of the Coolidge tube. The 

 amount of interference seemed to be dependent on the 

 frequency of interruption of this current. Proper pre- 

 cautions, however, made this trouble negligible. Several 

 series of observations were made in the manner des- 

 cribed. The frequency of the X-rays was varied through 

 a considerable range. Sensitivity tests before and after 

 the measurements showed that a resistance change of 

 one ten-thousandth ohm in the X-branch would cause a 

 deflection of twelve millimeters, Xo change in resist- 

 ance caused by the X-ray beam was detected. 



Fluctuations in the rate of change of temperature 

 made it impossible to take full advantage of the accuracy 

 of which the bridge used is capable. However it may be 

 said that any resistance change which does occur when 

 tin is subjected to such an X-ray beam is less than one 

 part in four hundred thousand, or less than 0.00025%. 



Experiments with more adequate temperature control 

 to permit much greater accuracy and with other elements 

 than tin are anticipated. 



In conclusion, we wish to thank Professor A. P. Car- 

 man for the facilities of the department. 

 Laboratory of Physics 



Universitv of Illinois 

 April, 1922. 



