290 Crooker— Influence of a Series Spark 



to 0-3 mm. several hundred sparks passed per second and 

 the corona tube was completely filled with streamers. 

 In every ease the sparks were unidirectional, sharp and 

 clean-cut showing no oscillatory character whatever. 



In order to confirm the results obtained by the Boys 

 method and to determine the form of the current curves 

 when a spark is in series a hot-lime-cathode Braun tube 

 was designed and constructed as shown in figs. 6 and 7. 



A narrow platinum strip P fastened to the insulated 

 brass blocks B x B 2 is heated by an auxiliary current pass- 

 ing through the leading-in conductors Cj C 2 . A small 

 spot of CaO placed upon this heated strip has a peculiar 

 property of giving off a stream of slow moving electrons 

 when used as cathode in a discharge tube at a very low 

 pressure. It is necessary to use only a low potential of 

 about 400 volts between anode A and cathode C. The 

 block holding the platinum strip was mounted upon a 

 gimbal support, as shown, in order that the soft cathode 

 beam could be easily adjusted through a hole in the dia- 

 phragm D, fall upon the fluorescent screen S and there 

 produce a spot of maximum brightness. This double 

 adjustment is necessary for it is impossible to assure by 

 construction the exact direction of the beam. 



If a very weak magnetic field is placed at right angles 

 to this beam of slow moving electrons the beam will be 

 deflected, shifting the bright spot on the fluorescent 

 screen. When the magnetic field is alternating or 

 pulsating the rapidly moving spot will cause a line to 

 be seen on the screen. Now if this line is observed iii a 

 mirror which rotates at right angles to it, the line is 

 spread out into a curve which represents the variations in 

 the current of the coil which excites the magnetic field. 



The coil used had about 3,000 turns of No. 26 enameled 

 copper wire wound in two sections and mounted so that 

 it could be fitted closely to the neck of the tube. 



It might be advantageous to note briefly some of the 

 details necessary , in constructing and operating the hot- 

 lime-cathode Braun tube. 



1. The cathode should be adjustable in order to get a 

 spot of maximum brightness. 



2. A diaphragm D is necessary to cut out extraneous 

 light from the hot platinum strip and to stop down the 

 divergent cathode beam. 



3. The cathode should be as near the fluorescent 

 screen as the sensitiveness of the apparatus permits. 



