814 Mr. H. Carter on the Radioactive 



Conclusions. 

 The curves obtained from the two blowpipes show 

 practically no difference between the two rates of leak. The 

 results are more conclusively negative than those obtained 

 from the arc, as a much smaller distance between the flame 

 and the cover of the electroscope could be used. 



Ike Electric Spark. 

 For these tests a 6-inch spark-coil with hammer break 

 run from an 18-volt supply was used. Brass rods were fitted 

 to the ends of the ordinary sparking terminals, to bring the 

 spark over the cover of the electroscope. A condenser was 

 placed across the spark-gap, which was about 1 mm. in length. 

 The same arrangement of electroscope and water-jacket 

 adopted in previous cases was used. In order to decrease 

 the distance between the spark and the cover of the electro- 

 scope a small hemispherical bowl of iron fitting in the hole 

 iu the water-jacket was used as the screen in the blank 

 experiments. The spark was kept on continuously, so that 

 the conditions of the two tests were exactly similar. 



The chief difficulty in this part of the work was found to 

 lie in the choice of a suitable cover for the electroscope. 

 The copper-foil used in the previous experiments and a thick 

 tinfoil mounted with cardboard showed no difference 

 between the rates of leak in the blank and shielded tests. 

 A thinner tinfoil and very thin aluminium leaf mounted 

 with cardboard were found to give way very quickly, probably 

 on account of hot particles of brass thrown off from the 

 electrodes and the impact of the spark. When working with 

 these defective covers very large rates of leak were obtained 

 in the exposed experiments, probably on account of the 

 entrance of ionized air into the case of the electroscope, and 

 due to the action of entladungstrahlen. 



Finallv, a more satisfactory cover was made by fixing a 

 sheet of aluminium leaf to each side of a square piece of 

 wire gauze, mounting this with cardboard, and waxing to the 

 case of the electroscope as before. 



To minimize the possibility of entrance of ionized air into 

 the chamber of the electroscope, the steel chimney used for the 

 other experiments was fitted, an auxiliary draught being- 

 provided by a small gas-jet burning at the end of a fine 

 glass tube inside the chimney. 



Shielded and exposed curves were now obtained as before, 

 in which there was no appreciable difference between the two 

 rates of leak. 



The distance between the spark-gap and the cover was 

 3-8 cm. 



