of Radium on the Electric Spark.- 379 



-ample Was not known, but in most of the experiments it was 

 not of the highest activity. 



The action was altogether different according to the 

 direction of the discharge. Using a spark-gap longer than 

 2 cms.j and making the larger knob positive, the radiations 

 had practically no influence. With the smaller sphere positive 

 the radium in most cases extinguished the spark, the visible 

 portion of the discharge being confined to a glow at the 

 surface of both electrodes. With the machine used a torrent 

 of sparks could be kept up with a gap o£ 3 cms. ; in 

 these circumstances th<> spark was least sensitive to the 

 radium. 



As the knobs were gradually pulled apart, a good deal of 

 brush discharge became mixed with the spark, and the effect 

 was more easily observed. With still longer discharges there 

 appeared only a positive brush, beyond this a dark space, 

 and on the surface of the cathode a faint glow. In this 

 condition the discharge is very susceptible to influence, it 

 may be extinguished instantaneously by the radium held at a 

 distance of 1 metre. 



The following series of observations and the accompanying 

 curve (fig. 1) plotted from the numbers, show how the 

 sensibility increases with the spark-length. The two spheres 

 were set apart so that their nearest points were distant from 

 each other the lengths given in the first column, the machine 

 was put in action and the radium gradually approached along 

 a line at right-angles to the discharge. The numbers in 

 column 2 give the distances at which the discharge was just 

 extinguished; in each case the numbers are the mean of 

 several ol >servations. 



Distance apart of knobs. 



Distance ol' radium from spark-gap 



Smaller sphere positive. 



when the discharge is just put out. 



cms. 



cms. 



4 



6* 



5 



12 



6 



15 



i 



17-5 



- 



20 



9 



24 



10 



28 



11 



2U-7 



13 



39 



15 



66 



* The discharge iu this case is mixed brush and spark. Most of the 

 brush is stopped at 10 cms., the spark also at cms. 



