694 Mr. C. Gr. Barkla on Secondary 



The conclusion was therefore that the tertiary radiation 

 from solids was negligible, so that the ionization measured by 

 the electroscope was produced by secondary radiation from 

 the gas inclosed in the gas chamber. 



As the intensity of the primary radiation was not constant 

 throughout the experiments, a second similar electroscope Z 

 was placed behind a leaden screen with a small hole X in it 

 opposite the thin face of this electroscope, and primary radia- 

 tion from the aperture A in the large box containing the 

 induction-coil and bulb passed through this small hole into 

 the electroscope. The rate of motion of the gold-leaf was a 

 measure of the rate of ionization produced in air by the 

 primary radiation, or for rays of the same kind it was a 

 measure of the intensity of primary radiation. The total 

 deflexions in a giyen time were then used to standardize the 

 intensity of primary radiation. 



The two charging-rods of the electroscopes were connected 

 to one terminal of a battery Y whose other terminal was 

 earthed. Momentary contact was made with the insulated 

 portions by means of the contact-makers and the gold-leaves 

 were deflected. Both electroscope-cases were earth-connected 

 so that a definite deflexion corresponded to a definite potential 

 of the gold-leaves. The deflexions were read by the micro- 

 scopes ^ and P. Readings were taken at intervals for 

 several hours in order to measure the constant fall due to 

 spontaneous ionization of the air in the electroscopes. The 

 induction-coil was then worked for thirty or, in some cases, 

 twenty seconds, during each whole minute for another period 

 — usually an hour. Readings were taken at intervals and a 

 constant proportionality between the deflexions of the gold- 

 leaves was found. The electroscope subjected to primary 

 radiation was then used to standardize the intensity of the 

 radiation proceeding from the bulb. 



TVhen another gas was experimented upon, everything 

 remained in position as before. A gas generator G was con- 

 nected to an inlet tube which was at the top or bottom of the 

 gas chamber according as the gas was lighter or heavier 

 than the gas which it was replacing. An outlet tube T con- 

 ducted the gas from the opposite end of the chamber. The 

 insulated portions of the electroscopes were recharged and 

 the same experiments repeated when the gas to be experi- 

 mented upon filled the box. The deflexions of the standardiz- 

 ing electroscope in a given time did not vary much, but those 

 of the other electroscope varied considerably with the gases 

 used, as the following table shows : — 



