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measuring slight and strong ionisation, since it combines the advantages of the 

 ordinary microscope observation in measuring slight ionisation, with the superiority 

 of the magnifying glass in measuring strong ionisation. Thus we can dispense with 

 the auxiliary condensers for varying the sensitiveness of the testing apparatus. 



The change in the deflection of the aluminium leaf is observed by the ordi- 

 nary methods of observation, but on account of the change in the position of the 

 leaf, the capacity of the electroscope is also slightly altered. By my method of 

 observation, however, the capacity of the electroscope remains quite unaltered 

 throughout a series of observations, because the deflection of the leaf is identical at 

 every observation. 



Owing to the special arrangement of the electroscope and the ionisation 

 chamber shown in Fig. 5, the measurements are quite independent of the moisture 

 of the atmosphere. For the only place where the insulation may be defective on 

 account of moisture is the surface of the amber stopper, and owing to the arrange- 

 ment of the apparatus, this may easily be dried by means of metallic sodium. 

 The electroscope vessel is provided with a side tube, in which the sodium is put 

 when the upper surface of the amber is to be dried. The lower surface of the 

 stopper is most convenientl}' dried by separating the electroscope vessel from the 

 ionisation chamber, and closing the bottom of the vessel by the previously men- 

 tioned cover, in which a small piece of sodium is placed beforehand. On account 

 of the small space to which the action of the sodium is confined the surface of 

 the amber is quite dry after a few minutes. 



In the first measurements of radioactivity on our journey, we noticed some 

 remarkable anomalies in the results, but as all the testings w'ere carried out in 

 a tent, where considerable variations in the temperature are liable lo occur, we 

 soon perceived that these anomalies were due to air currents set up by the varia- 

 tions of the temperature in the electroscope vessel, the deflection of the mobile 

 aluminium leaf being altered by the slightest movement of the air in the electro- 

 scope. In order to gel rid of these disturbances, the electroscope was enveloped 

 in a coating of thick pasteboard, covered with tin-foil. This isolation of the 

 electroscope against heat proved to be sufficient to uullify the effect of the changes 

 in the temperature. 



When the emanation in a gas was to be mea.sured by means of the above 

 described apparatus, I first determined the amount of leakage through ordinary 

 atmospheric air, then a measured quantity of the gas to be examined was intro- 

 duced into the ionisation chamber. The gas was filtered through a compressed 

 plug of cotton wool which retained the disintegration products present in the 

 radium emanation (Ra A, Ra B, Ra C). 



The time of the introduction of the radioactive gas into the ionisation vessel 

 was noted. Then the leakage was tested several times in the course of an hour, 

 and from these measurements the amount of radium emanation introduced with 

 the active gas into the ionisation chamber was calculated. 



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