33 G Prof. W. H. Bragg on the Ionization of 



spread on one of its walls. For in this case some of the 

 particles would complete their full ranges within it, others only 

 part, and an estimate of the ionization to be expected would 

 render it necessary to take into account the amount of the 

 range completed by each particle as determined by the nature 

 and physical conditions of the gas and the dimensions of the 

 chamber, the reckoning being further complicated by the fact 

 that the ionization produced by the particle is not constant along 

 its path. It is possible that an experiment might be arranged 

 in which a thin sheet of a. radiation entered the chamber 

 through a slit at the side, and spent itself within the chamber 

 without touching the walls. It would be necessary to make 

 sure that the same portion of the range was completed within 

 the chamber by the particle, no matter with what gas the 

 chamber was filled. I have not yet tried this plan. 



It will now be clear, I think, that the method actually 

 used is not without its advantages. It avoids the use of very 

 high potentials, and does not require lengthy and uncertain 

 calculations. It has also this in its favour, that it gives the 

 range of the particle in the gas, so that it is possible to make 

 a sufficiently accurate estimate of the amount of any air that 

 may be present. The presence of this air can then be 

 allowed for. 



Let us, therefore, proceed to consider the assumptions and 

 approximations which the method requires. 



In the first place it is necessary to consider whether any 

 disadvantages are likely to arise from the use of a sheet of 

 gauze as the lower wall of the ionization-chamber. The electric 

 field must be distorted in the neighbourhood of the gauze ; 

 some very small portions of the chamber which are just over 

 the openings in the gauze must be under feeble forces, and 

 the ions made there be separated only when the potential is 

 high. It is easy, however, to show that this effect is negligible 

 Vf a consideration of the ionizations due to 13 rays. This 

 ionization does not show initial recombination, as in the case 

 of the a rays; a fact first demonstrated by R. Kleeman, 

 formerly of this University. In fig. 2 are drawn the upper 

 parts of the ionization curves of ethyl chloride under different 

 potentials. It will be seen that in the portion which repre- 

 sents the effects of /3 rays only, saturation is complete when 

 50 volts are applied to the 3 mm. chamber ; but the ionization 

 due to * rays is far from being collected completely by ten 

 times that potential. Now, if the field distortion due to the 

 gauze were appreciably effective, we should find the j3 rays 

 also producing an ionization which appeared to increase at 

 higher potentials ; and there is no trace of any such effect. 



