20 PHYSICAL MEASUREMENT OF X-RAYS. — BRONSON. 



the testing vessel was so high in comparison that R can be 

 neglected in the calculations. ^I may be anj' fairly steady 

 source of potential, high enough to produce practical satura- 

 tion. It should be at least 100 A'olts and was about 200 in 

 the present experiments. Sometimes it was obtained from 

 storage cells and sometimes from a small 220 volt D. C. 

 motor used as a dynamo and driven by a small A. C. motor. 



One potential terminal was connected to the outside 

 lead covering of the testing vessel and the other to the insu- 

 lated aluminum foil E. The foil F was used to protect E 

 from possible electrostatic action, and both were too thin 

 to produce appreciable absorption of the X-rays. The 

 aluminum plate C was very carefully insulated on sealing 

 wax so that the galvanometer would be sure to measure the 

 current due to the X-ray ionization between the plates E 

 and C. The wire leading from C to the galvanometer was a 

 small lead covered cable. The lead cover was connected 

 through the high resistance R to one terminal of the galvano- 

 meter and to the lead case of the testing vessel, thus com- 

 pleting the circuit. 



The X-ray tube was mounted at any desired distance 

 directly above the opening F. The size of the bundle of 

 rays entering the vessel was determined by placing on top 

 of the vessel a lead sheet with suitable opening. Six tubes 

 of various makes and construction were used and similar 

 results were obtained with all. The smallest tube was 

 about 13cm. in diameter and had a light platinum target, 

 and the largest was about IScm. in diameter and had a 

 heavy tungsten target. 



The different sources of power used included a large and 

 a small induction coil with both Wehnelt and mechanical 

 interrupters and a high tension transformer and commutator 

 (Waite and Bartlett Mfg. Co.), giving an interruptcrless 

 current. The current through the tube was measured by 

 a milliammeter, but the spark gap was the only means at 



