Secondary 7 Radiation. 425* 



can be placed horizontally as at e. When increasing the 

 thickness of radiator the top plate is always kept in the 

 position e, and additional thicknesses are placed immediately 

 below and in contact with it. When large thicknesses are 

 required, pings of the material may be inserted in the 

 conical hole. 



The ionization-chamber was filled with ethyl chloride :. 

 this increased the effect considerably and worked very satis- 

 factorily. 



It was necessary 'to balance the leak due to natural ioniza- 

 tion in the chamber, and to 7 radiation which was not 

 complete^ absorbed by the lead block and plates. For this 

 purpose a balance chamber of about the same size as the 

 ionization-chamber was placed against one of the sides of 

 the iron case insulated from it. 



This was connected up in the usual manner, and was 

 found to give quite sufficient balance with the ionization 

 produced by the 7 radiation which came through the sides 

 of the lead block containing the Ha. The electrometer was 

 a sensitive instrument of the Dolezalek pattern, giving a 

 scale reading of about 4.000 divisions per volt ; each division 

 could easily be subdivided to tenths. The charge was allowed 

 to pass into the electrometer for 30 sec, and the mean of the 

 first and second swings was taken as the deflexion. 



In fig. 2, curve A shows the effect of increasing the 

 thickness of a Pb radiator. In an experiment, the zero leak 

 was first obtained as a mean of several determinations, and 

 this was subtracted from the measurement obtained with the 

 radiator in position. It will be seen from the curve that 

 the effect increases until a thickness of between 5 and G mm. 

 of Pb is reached, after which it decreases gradually. 



If, now, a hemispherical dome of Pb 4 mm. thick, with 

 its central portion removed so as not to intercept the original 

 stream of 7 rays, be placed in the position shown by hji, 

 fig. 1, and a set of observations be taken as before, sub- 

 tracting the reading with the radiator in from that with it 

 not in position, the curve B is obtained. Again, a maximum 

 is reached for about the same thickness of radiation as before, 

 but the effect is for the different thickness of radiator about 

 (50 per cent, of its previous value. We may conclude that 

 it is 7 radiation which is being intercepted by the dome, as 

 in neither case can any but a very small amount of ft radia- 

 tion pass obliquely through the thickness of 1*25 mm. of lead 

 forming the sides of the ionization-chamber : and that from 

 the radiator are proceeding secondary emergence 7 rays 



