Experiments on Residual Ionization. TO 5 



temperature hal to be reduced considerably. The zinc 

 ionization-ehamber previously described was used. It was 

 covered with thin asbestos, then wound with nichrome 

 resistance-wire, and packed in magnesia. By passing 

 currents up to 1*5 amperes through this wire, the chamber 

 could be maintained at any temperature between 10° and 

 100° 0. for as long a time as desired. The chamber was 

 absolutely airtight, the wax joints around the electrode 

 and guard-ring being kept cool with a water-jacket. 

 The temperatures were calculated from the changes in 

 pressure. 



Variation of Ionization with Pressure. — The gases used 

 were carbon dioxide, acetylene, and hydrogen. Several sets 

 of readings were taken with each of these oases at room 

 temperature to show the conneKion between ionization and 

 pressure. The readings and curve (fig. 5) shown were 

 obtained with carbon dioxide, and are typical of the others. 

 The ionization shows a slight maximum at (^50 mm. pressure, 

 due presumably to a soft radiation from the walls of the 

 chamber. 



p (mm.). 



■ 



764 



17-n 



726 



166 



1 



" 



626 



in o 





16-0 



519 



14 -J 



450 



131 



366 



11-6 



. . 



- 



141 



5*5 



Co 



: 6 



We may proceed as follows to see whether this curve 

 gives any indication of the presence of ionization by collision. 

 The possible components of the ionization are — (1) the ioni- 

 zation due to the earth's penetrating radiation, which from 

 the experiments of McLennan and Treleaven will be about 

 5'1 ions per c.c. per second at 760 mm. pressure: (2) that 

 due to a possible ionization by thermal collisions, which from 

 the same experiments cannot be more than about 4'8 ions 

 per c.c. per second at 760 mm. pressure: (3) that due to any 

 radioactive impurity in the walls of the receiver. Xow 

 component (1) will vary directly ;ts the pressure, and may be 



