246 I E. Kuthertunl ami Mr. II. K. .MrClun- 



The values of H and i are experimentally determined, and, assuming 

 the value of , namely, 6*5 x 10~ 10 electrostatic unit, determined by 

 J. J. Thomson, the value of W is found in absolute measure. 



In the course of the investigation the following subjects have been 

 considered : 



(1) Measurement of the heating effect of X rays. 



(2) Efficiency of a fluorescent screen excited by X rays as a source 

 of light. 



(3) Absorption of X rays in gases at different pressures. 



(4) Determination of the energy required to produce an ion in air 

 and other gases, including deductions on 



(") Distance between the charges of ions in a molecule. 



(b) Minimum potential difference required to produce a spark. 



(5) Energy of Becquerel rays and emission of energy by radio-active 

 substances. 



Heating Effect of X Hays. 



An automatic focus tube was employed, excited by a large induction 

 coil with a special form of Wehneldt interrupter giving fifty-seven 

 breaks per second. The bulb gave out intense rays of a very pene- 

 trating character. 



The heating effect was measured by determining the variation of 

 resistance of a special platinum bolometer when the rays fell upon it. 

 A platinum strip, about 3 metres in length, 0'5 cm. wide, and 0'003 cm. 

 thick, was wound on a light mica frame 10 cm. square. Two such 

 " grids," as similar as possible, were constructed, and formed the two 

 arms of a Wheatstone bridge. A balance was obtained for a momentary 

 pressing of the battery key, using a sensitive galvanometer. The raya 

 were then turned on for 30 or 45 seconds, and the deflection from zero 

 determined immediately after the rays were stopped. 



In order to measure the heating effect, a current was sent for the 

 same time as the rays acted through the grid, and its value adjusted 

 until the deflection due to the heating of the grid was the same as for 

 the rays. When this is the case the heat supplied per second to the 

 grid by the rays is equal to the heat supplied per second by the 

 current. 



Thus, heating effect of rays per second = 0'24i- R calorie, where 



t = current through the grid of resistance R. 



The grids were enclosed in a lead vessel with an aluminium window 

 to let in the rays. The whole was surrounded by a felt covering, and 

 several aluminium plates intervened between the bulb and the grid, so 

 that any heating effect, except that due to the rays, was completely 

 eliminated. 



