■334 Messrs. Moseley and Robinson on the Number of 



was found at a distance of 3 metres to contribute 5 per cent, 

 to the ionization. Since Eve always used a magnetic field 

 as well as a screen to stop the j3 rays, this accounts for a 

 small part of the divergence between our results. We pro- 

 duced the magnetic field by means of a large electromagnet 

 with poles so far apart that a clear path was left for the 

 7 rays throughout a cone of more than 50°, but even so the 

 presence of the electromagnet when unmagnetized diminished 

 the ionization at 3 metres by 16 per cent. This result is 

 scarcely surprising, seeing that the particles which give 

 rise to the ionization are themselves excited in the air all 

 round the ionization chamber and at a distance up to several 

 metres from it. It is difficult to arrange an electromagnet 

 so that the 7 radiation is unaffected throughout the region in 

 which the operative /3 particles are generated, and an explan- 

 ation of the divergence between our results and those of Eve 

 may possibly lie in this direction. 



Before using equation (3) to calculate N, it is necessary to 

 enquire whether equation (2) really holds in this case. It is 

 obviously not true at short distances ; for it gives very large 

 values of n in the neighbourhood of the source, a region in 

 which few /3 particles, except those generated in the alu- 

 minium, will have been excited. As these few appear 

 initially to follow the direction of propagation of the 7 

 radiation, the value of nr 2 excluding the effect from the 

 aluminium will here be small. In the extreme case in 

 which the secondary /3 particles continue precisely in this 

 direction, 



nr 2 =Ke~* r {l—e~ kr ), 



where X is the absorption coefficient of the secondary /3 radi- 

 ation. Here nr 2 vanishes in the neighbourhood of the source. 

 In practice these simple conditions are not satisfied, and it is 

 impossible to calculate the variation of nr 2 at short distances. 

 At 3 metres, however, the effect cannot be large, and is 

 roughly balanced by the radiation from the aluminium, 

 which itself only accounts for 5 per cent, of the ionization. 

 At greater distances the effect of scattered 7 radiation from 

 the walls and floor of the laboratory may perhaps become 

 appreciable, so that, as already remarked, we have adopted 

 the value of nr 2 at 3 metres in all our calculations. 



We have seen that this value led to the results 

 K = 4'89 x 10 9 for the 7 rays of radium C, and K = 1*61 x 10 9 

 for the 7 rays of radium B. Now the value of yu, found by 

 Chadwick for air at 15° C. and 76 cm. pressure, after 

 the 7 rays had passed through 1 cm. of lead, was 

 -000057 cm." 1 



