on the Ions produced in Air by Rontgen Rays. 431 



always a danger that some of the drops in the cloud contain 

 more than one ion. 



The apparatus used is shown in the accompanying diagram. 



It consisted of a glass tube AB about 4 cms. in diameter 

 and 10 cms. long. Its lower end was closed by an india- 

 rubber stopper and its upper end joined on to a short length 

 of narrow tubing. Two circular brass disks, C and D, each 

 3*5 cms. in diameter, were supported one above the other in 



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en 



= lHP== r^^^?]J] y, ro 



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/AlV£CTOK POVfr 



this tube as shown ; the cloud on which the observations 

 were made was formed between them, and they could be 

 maintained at any required difference of potential up to 

 2000 A^olts by means of a battery of small secondary cells. 

 A glass tube E was sealed on to the side of AB and served 

 to connect the space between the disks with an apparatus for 

 producing a sudden expansion of any desired amount. A 

 small mercury manometer (H) was used to measure the 

 expansion, 



