the Cause and Nature of Radioactivity . 375 



increases uniformly with time, and the time taken to pass 

 over 100 divisions of the scale is taken by a stop-watch. 

 The rate of movement is a measure of the ionization-current 

 between the plates. The ratio of the currents for different 

 substances is a comparative measure of their radioactivity, 



"With this apparatus *5 gr. of thorium oxide produces a 

 current of l'lx 10~~ n amperes, which, with the electrometer 

 used, working at average sensitiveness, corresponds to 100 

 divisions of the scale in 36 seconds. In certain cases a special 

 modification of the Dolezalek electrometer was employed 

 which is 100 times more sensitive. With this instrument 

 the radioactivity of 1 milligram of thoria produces a 

 measurable effect. If the substance gives off an emanation, 

 the current between the plates increases with time. Under 

 these conditions, when the thorium compound is exposed in 

 thin layers with a maximum of radiating surface, all but one 

 or two per cent, of the total effect is due to the straight-line 

 radiation. Even when the effect due to the emanation has 

 attained a maximum, this constitutes a very small fraction of 

 the whole. This effect, however, may to a large extent be 

 eliminated by taking the current between the electrodes 

 immediately after the material is placed in the testing- 

 apparatus. It may be completely eliminated by passing a 

 current of air between the electrodes to remove the emanation 

 as fast as it is formed. 



The current between the plates observed with the electro- 

 meter at first increases with the voltage, but a stage is very 

 soon reached when there is a very small increase for a large 

 additional voltage. A P.D. of 300 volts was sufficient to 

 obtain the maximum current, so that all the ions reached the 

 electrodes before any appreciable recombination occurred. 



It must, however, at once be pointed out that it is difficult 

 to make any absolute measure of radioactivity. The radiation 

 from thorium is half absorbed by a thickness of aluminium 

 of "0004: cm. ; and since thorium oxide is far denser than 

 aluminium, it is probable that the radiation in this case is 

 confined to a surface-layer only '0001 cm. deep. It is obvious 

 that different preparations, each containing the same per- 

 centage of thorium but with different densities and different 

 states of division, will not give the same intensity of radiation. 

 In comparing two different specimens of the same compound, 

 it is important that the final steps in their preparation should 

 be the same in each case. As a rule absolute measurements 

 of this kind have been avoided. It is possible, however, to 

 trace with great accuracy the cltange of radioactivity of any 

 preparation with time by leaving it undisturbed on its 



