i8 



SCIENCE PROGRESS 



meter or electroscope, is no more, for the present at least, than 

 an indicator of where the rays have been at work and a measurer 

 of what they have done. It is this method which is used in the 

 experiment I shall now describe. 



In a report presented to the International Congress at Paris 

 in 1900 an experiment is mentioned in which some radium had 

 been placed close to an ionisation chamber with a wide opening 

 through which the rays streamed in. The effect of the rays, 

 as might be expected, varied with the distance of the radium 

 from the chamber but there was a somewhat sudden alteration 



T 



m. 



A 



Fig. 1. 



in the effect when the distance of the radium from the chamber 

 was increased beyond a certain limit. As Mme. Curie says 

 (see the figure which represents part of her apparatus), " For 

 a certain value of the distance a t (at least 4 cm.) no current 

 passes {i.e. there is no current in the chamber p p p' p' and 

 therefore no ionisation there) : the rays do not penetrate into 

 the condenser {i.e. the chamber). When the distance at is 

 diminished the appearance of the rays in the condenser comes 

 about suddenly so that a small diminution of the distance turns 

 a very feeble current into a strong one : after that the current 

 grows regularly as the radiant body [Mine. Curie was using 

 polonium] approaches the window t." 



