PAPERS ON CHEMISTRY AND PHYSICS 335 



hot for an instaut. After tile beam was started the 

 cathode was rotated until it was projected against the 

 side of the tnbe. When the current was turned on in the 

 solenoid circuit, the beam was caught in an intense mag- 

 netic field and wound into a spiral which traversed the 

 length of the discharge chamber, striking on a willemite 

 screen on the outside of the camera shutter. 



The intensity, size, shape and position of the phos- 

 phorescent spot could be changed by adjusting or regu- 

 lating the pitch of the cathode ray spiral, the tempera- 

 ture of the hot cathode and the solenoid current. A 

 focusing coil enabled the final adjustment to be made, 

 after which a series of photographs were taken. 



Six photgraphs were taken on each plate. A practice 

 was made of taking the odd numbered exposures with- 

 out, and the even numbered ones with the radiation fall- 

 ing on the electron spiral. The time betw^een exposures 

 was five to six seconds. 



IV. MEASUKEMENTS 



After the photographic plates had been developed and 

 numbered, they were carefully examined to see which 

 ones could be subjected to measurements. A plate, to 

 be of value for measuring, had to possess certain quali- 

 fications adopted as standard. First, the electron trace 

 had to be of sufficient intensity to be seen easily with 

 the naked eye, since faint traces could not be seen under 

 the microscope used in measuring the photographs. Sec- 

 ond, the trace had to form an arc of a circle of sufficient 

 length to measure its diameter. Third, the edges of the 

 circle had to be sharp so that the error in measurement 

 might be small. Fourth, the six traces on a plate had 

 to be similar so that corresponding measurements could 

 be taken on each one. 



A short table has been prepared as typical of the total 

 results from data taken from consecutive plates. In this 

 series the electron beam making the traces was alter- 

 nately exposed to hard X-Eays. Fig. 2 shows Plate No. 

 720 listed in Table I. 



