334 ILLINOIS STATE ACADEMY OF SCIENCE 



the moving electrons. Two sources of radiation were 

 used, — a ninety degrees carbon arc, and a Coolidge 

 X-Ray tube. Tliey were placed about 40 cm. from the 

 beam of electrons upon which the radiation was to fall. 

 The X-Eay tube used was the universal type Coolidge 

 tube with a broad focal spot. This was excited by a 

 Klingelfuss induction coil with a six-inch spark gap. 

 The electron beam was maintained by a potential differ- 

 ence of about 2,000 volts from a small high potential 

 storage battery. 



A camera was placed within the discharge chamber 

 at the end opposite the cathode an-d connected through 

 a ground glass joint to the outside in such a way that 

 the photographic plate could be rotated, making it pos- 

 sible to take six exposures on a single plate. The camera 

 shutter was operated by a magnet, but was designed 

 so that the time lag between the tripping and the open- 

 ing of the shutter permitted the magnetic effects of the 

 operating magnet to disappear before the photograph 

 w^as taken. 



III. OPEKATING CONDITIONS 



Due to the fact that the experimental operations of this 

 research were very critical, the exact conditions imder 

 which the results w^ere obtained are definitely stated. 

 The vacuum was always 0.00001 mm. of mercury or less 

 when the exposures were started. At the end of a series 

 of exposures the pressure was measured and it was sel- 

 dom higher than at the beginning. The discharge cham- 

 ber was freed of water and mercury vapors by a P2O5 

 bulb, a large charcoal bulb, and a liquid air trap, the last 

 two being immersed in liquid air. 



The Wehnelt cathode was heated to a temperature 

 gained by experience until a beam of electrons of suffi- 

 cient intensity was obtained to make an impression on 

 the photographic plate. Because of the high vacuum 

 used and the absence of any trace of mercury vapor, it 

 was sometimes very difficult to start the discharge even 

 on the application of 2000 volts. It could, however, 

 usually be induced to start by heating the cathode very 



