162 Mr Cooke, Experiments on Penetrating Radiation. 



the hole bored down the centre. The drying agent used is calcium 

 chloride, wrapped in four small linen bags, resting in porcelain 

 trays, and kept in position by means of an electrostatic screen, 

 soldered to the inside of the cylinder. In setting up the vessels, 

 of which there were two, great care was taken in preserving the 

 utmost cleanliness and freedom from contamination with radio- 

 active materials. Except for the central rod, all metal parts were 

 of brass, the sides and top being about 2 mm. in thickness. All 

 metal to metal joints were soldered, the others being made with 

 a marine-glue preparation. The diagram is drawn to scale. The 

 volume of ionized air is 510 c.c. 



The method of using these vessels was as follows ; the case 

 being supported on a stand connected to earth, the central rod 

 was charged to an accurately measured potential, usually between 

 400 and 410 volts. The top cap was then replaced and waxed 

 down, and the vessel set in the place where the ionization was to 

 be measured. After remaining there for several hours it was 

 placed on an insulated stand, connected with one pair of quadrants 

 of a Dolezalok electrometer. After separating the quadrants the 

 central rod was charged by means of the plunger to the original 

 voltage. This resulted in the central rod acquiring a quantity 

 of electricity equal to that carried away by the total ionization 

 current during the experiment, and an equal charge of like sign 

 being repelled to the quadrants of the electrometer, and measured 

 by the deflection. 



The object in making two similar vessels was to eliminate if 

 possible the effect of any diurnal variation in the natural ioniza- 

 tion, by always exposing one under similar conditions, the other 

 being reserved for measuring the leak under varying circum- 

 stances. It was soon found, however, that the ratio of the two 

 ionization currents under apparently identical conditions was not 

 more constant than the currents individually. After this was 

 shown to be the case both vessels were utilized in examining vary- 

 ing conditions, the one being a check on the other. 



It was originally intended to send the vessels up attached to 

 a balloon or kite, but this plan was abandoned owing to the 

 difficulties and danger of operating in this manner in the neigh- 

 bourhood of a large town. The method first adopted was to 

 experiment with the vessels on the roof, shielding with lead 

 screens, first from above and then from below. Experiments were 

 carried on in this manner for a considerable time, but no reliable 

 results were arrived at. There were strong indications however 

 that the radiation was more intense from below than from above. 

 The measurements showed a reduction of about 12% when the 

 apparatus was shielded from below and from the sides by being 

 placed in a cup-shaped lead-casting one inch in thickness, When 



