3?8 



Scientific Proceedings, Royal Dublin Society. 



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this will be important if the vessel is small. (3) Ions may collect molecules 



or dust-particles, forming what are known as large 

 ions. Tliis loss of conductivity of an ionised gas is 

 made use of in the method of experimenting. 



Fig. 1 shows the general arrangement of the 

 apparatus. A tube AB, usually of glass, fits into 

 a larger metallic tube BC, from which another exit 

 tube C leads to (1) an apparatus for measuring 

 the volume of air (if required) ; and (2) to an ap- 

 |l|l|l |l|i I paratus {e.g. electric fan or gasometer) for drawing 



the air through. 



It is in AB that the flow of air is to be studied. 

 The vessel BC is insulated and connected to the 

 negative pole of a battery (Z>) of some two hundred 

 volts. The positive pole of the battery is earthed. 



Through an insulating plug {H) in the side of 

 BC a wire E passes out; this is joined to one pair 

 of quadrants of a Dolezalek electrometer (F), and to 

 one side of a variable condenser [K). The other 

 electrometer quadrants and the other side of the 

 condenser are, of course, earthed. The key L allows 

 of the wire E and its connexions being earthed 

 when required. The radioactive substance (radium 

 bromide) is placed at the end A of the tube ; and 

 its rays are screened from the rest of the tube by 

 a lead block placed as shown. 



On air being drawn through the tube in the 

 direction ABC, it is rendered highly conducting on 

 passing the radium at A ; but it gradually loses this 

 conductivity as it moves down the tube away from 



A. On arriving in the chamber BC — the ionisation 

 chamber — all the small negative ions, and perhaps 

 a few of the large ions, are driven into the wire 

 E, conveying a charge thereto. This charge is a 

 measure of the ionisation of the air on reaching 



B, and is measured by the rate of motion of the 

 needle of the electrometer. The ionisation is thus 

 expressed as so many divisions on the electrometer 

 scale per minute. When turbulent motion sets in the 



elements of the flowing gas commence to follow sinuous paths, so that a certain 



CD 



Fig. 1. 



