92 Mr. Richardson on the Positive Ionization produced 



Activit'/ Induced by Hot Wires at Hiaher Pressures. 



Practically all the preceding experiments with the two- 

 wire system were made at pressures of about a thousandth of 

 a millimetre. In order to see if the effect was obtained at higher 

 pressures a glass tap was added to the apparatus so that any 

 desired quantity of gas could be introduced. The induced 

 activity was very marked at 'O'"*^ mm. ; since heating the 

 second wire for 2 mms., the first being kept at —200 volts, 

 produced an initial current of 318 scale-divisions per minute. 

 This induced activity was found to die away much more 

 slowly than that produced by air and measured at a lower 

 pressure, as the following numbers indicate : — 



4-ve Leak zc . 



Time. 





min, sec. 



173 







14.5 



3 1.5 



11.5 



6 30 



91 



9 



70 



12 



58 



15 



U 



24 



22 



32 



12-5 



4.5 



8 



70 



3-5 



130 



The logarithms of the values of the induced leak, obtained 

 by subtracting three from the numbers in the above table, 

 are plotted against the time in fig. 6. It will be observed that 

 all the points fall very nearly in a straight line, showing 

 that the transferred activity follows the same law of decay 

 as the original activity of the wire. 



The hio-h values of the induced activitv oiven above were 

 now obtained every time. A repetition of the experiment, in 

 which the second wire was only heated for one minute, gave 

 an initial leak of 200 divisions in 15 seconds. 



The pressure was now reduced to "055 mm., and the 

 experiments repeated. Heating the second wire for thirty 

 seconds was now found to give an induced activity of 332 

 divisions per minute. The way in which the current fell ofi:" 

 with the time is shown in the followino[ table : — 



