24 PHYSICAL MEASUREMENT OF X-RAYS. 



By making such a comparison once for all, and using 

 aluminum instead of lead, it would be possible in a few 

 seconds by a single reading of the galvanometer, with proper 

 thickness of aluminum over the testing vessel, to determine 

 the time of exposure necessary to obtain a satisfactory 

 radiograph of any part of the body. Of course, this would 

 be of no practical value for powerful installations, where 

 exposures are for only a second or two, but might be of great 

 value where exposures for a considerable time have to be 

 made. 



(2) 



In order to investigate the second point, a Sabouraud 

 pastille was placed on the aluminum foil at F and exposed, 

 sometimes to the direct action of the X-rays and sometimes 

 with slight aluminum screening. During the time of exposure 

 the deflection of the galvanometer was read at regular intervals 

 The average of these readings multiplied by the time of 

 exposure and by the galvanometer constant gave the charge 

 that passed through the galvanometer, and this divided by 

 the volume of air ionized and by the number representing 

 the change in color of the pastille, as measured by Dr. Hamp- 

 son's radiometer, should give a constant K, if the effect 

 on the pastille is proportional to the ionization produced in 

 the testing vessel. The last nine comparisons gave the 

 following values for K: 2.4, 2.6, 2.7, 2.4, 2.5, 2.6, 2.4,2.4, and 

 2.6, all multiplied by 10^ Thus, we see that this method 

 furnishes an accurate and quick method of testing the thera- 

 peutic action of any tube. In these experiments only the 

 interrupterless current was used, but hard and soft tubes 

 were tried with currents varying from 1 to 5 milliamperes. 



(3) 



Less attention was paid to the measurement of hardness 

 than to the previous parts of the work, but a numl)cr of 

 experiments were tried to see how the ionization current 



