174 Mr Kleemav, The nature of the ionisation 



tissue paper, equivalent in mass to a layer of air 1 cm. thick. 

 The chamber was placed on the poles B^ and ^3 of an electro- 

 magnet, which Avere resting on a lead block C 5 cm. thick. This 

 lead block had an aperture a 3 cm. by 3'2 cm., which was placed 

 in a symmetrical position with respect to the poles of the electro- 

 magnet and the ionisation chamber. D is the tube containing 

 the radium placed at a distance of 10 cm. from the lead block. 

 The electrode of the chamber consisted of a wire bent into the 

 form of a square so that the principal stream of 7 rays through 

 the aperture did not impinge upon it. The ionisation in the 

 chamber was, as before, principally due to the direct ionisation 

 of the gas by the 7 rays and the secondary /3 rays from the plate 

 h placed over the aperture a. The plate h was of aluminium 

 4 mm, thick. 



The readings were carried out in exactly the same way as 

 in the foregoing experiment. The ionisation in the cone c 

 decreased to about 55 °/^ of its original value, when a magnetic 

 field of sufficient strength to prevent the /3 rays from the plate 

 h entering the chamber was applied. This remaining ionisation 

 is almost entirely due to the action of the 7 rays on the air in 

 the cone c, the ionisation by the secondary radiation from the 

 tissue paper being negligible since the paper is equivalent in mass 

 to less than a third of the air in the chamber while the ionisation 

 by the penetrating radiation from the air in a chamber of ordinary 

 size is negligible. 



The returned cathode radiation from the air outside the 

 chamber may for our purpose be neglected in comparison with 

 the radiation from the aluminium plate h. For if we assume 

 that the air is equivalent to a carbon plate giving the maximum 

 amount of radiation placed on top of the chamber, the returned 

 radiation is about -^ of the emerging radiation*. In the case 

 of air it will be less than that, since it radiates to a larger extent 

 sideways than the carbon plate. Now, at least one half of the 

 radiation emerging from the aluminium plate h enters the cham- 

 ber, and if we take the amount of emergence radiation of a plate 

 of carbon and aluminium the same, which is approximately true, 

 the returned radiation from the air is less than one-sixth of the 

 radiation from the aluminium plate. 



It appears therefore from these experiments that the ionisation 

 in a chamber is due in part to the direct action of the 7 rays on 

 the gas it contains. And since Laby and Kaye have shown that 

 the amount of ionisation produced by the secondary 7 and jS 

 radiation from the gas is small, this ionisation consists of slow- 

 moving ^ rays ejected by the 7 rays, which have not sufficient 

 velocity to produce any further ionisation themselves. 



* Bragg and Madsen, Trans. R. Soc. of South Australia, Vol. xxxii. p. 4, 1908. 



