352 Mr. C. A. Sadler on 



to —240 volts. The corpuscular radiation from copper 

 excited by Sr and Ag respectively was usea for measurement 

 purposes, but no appreciable change in the absorption co- 

 efficients in air could be detected. It is evident then that the 

 repulsion of the corpuscules by the plate R 2 in the one case 

 and their attraction by the plate in the other is too small to 

 affect their velocity to a measurable extent. 



It was reasonable to suppose, however, that the value of R 

 might possibly depend, in part, upon the sign of the voltage 

 applied to the ionization-chamber I. 



For consider first the case when the ionization-chamber is 

 charged to +240 volts. 



When the plate E 2 is earthed it acquires a negative charge. 

 On insulation it still remains at zero potential, and possesses 

 the same charge. If now a secondary beam of Rontgen rays 

 of given intensity pass into the ionization-chamber for a given 

 length of time, a certain amount of ionization will be produced. 

 The positive and negative ions will be equal in number. The 

 positive ions will be attracted to the plate and will diminish 

 its negative charge. This will be true whether the ions are 

 produced by the corpuscular radiation or by the incident 

 secondary Rontgen radiation in its passage across the ioni- 

 zation-chamber. But the emission of negative corpuscules 

 by the plate will of itself tend to diminish the negative charge 

 on the plate, so that from both causes the negative charge 

 will diminish and the potential of the plate and its con- 

 nected system will rise correspondingly. This change of 

 potential of the system causes a motion of the gold-leaf in the 

 electroscope E 3 , and by observing the deflexion of this leaf 

 we obtain a measure of the change of potential of the system. 

 This gives us a measure of the charge received by the 

 system on some purely arbitrary scale. 



Consider now the case when the ionization-chamber is 

 charged to — 240 volts. When the plate R 2 is earthed and 

 then insulated it possesses a positive charge equal in mag- 

 nitude to its previous negative charge. If we allow the 

 same secondary beam to enter the ionization-chamber for the 

 same length of time as in the previous case, an ionization 

 will be produced by the radiation of the Rontgen type equal 

 to that obtained in the former case. We have seen that 

 altering the sign of the potential of the ionization-chamber 

 produces no measurable change in the velocity of the cor- 

 puscles, and it is reasonable to suppose that the ionization 

 which they produce will be the same as before. Thus the 

 ratio of the total ionization due to the corpuscular radiation, 

 to that produced by the secondary exciting beam in 1 cm. of 



