434 Prof. McLennan and Mr. Found on Delta Badiation 



conjunction with the coconut charcoal cooled with liquid 

 air. Metallic zinc placed in F was brought to the boiling 

 point with the heating coils, and the rod H was lowered so 

 that the zinc plate X was directly above the opening in F 

 and immersed in the issuing vapour. With this arrange- 

 ment the zinc plate could be readily coated with a fresh 

 surface when desired. In studying the delta radiation from 

 this plate the rod H was raised with the windlass W until 

 the projection S was in electrical contact with the cup 0. 

 Under these conditions the zinc plate X was directly in 

 front of the polonium-coated plate P and was subjected to 

 bombardment by the alpha rays which were emitted by the 

 latter. 



It should also be mentioned that when in operation the 

 tube was set up with that portion about P in the field and 

 between the poles of an electromagnet. 



III. Experiments. 



Experiment I. — In commencing the investigation two 

 experiments were carried out similar to those described by 

 Logeman * in his paper on the emission of electrons from 

 metals bombarded by alpha rays. In the first experiment a 

 heating-jacket was placed about the tube containing the 

 charcoal so as to drive the air out of the latter, and the 

 apparatus was exhausted as highly as possible with a Gaede 

 rotary mercury-pump. After this was done the heating- 

 jacket was removed from B', and when the latter had 

 dropped to room temperature it was surrounded with a 

 Dewar flask and cooled with liquid air. When this was 

 done a McLeod gauge attached to the apparatus showed that 

 the pressure in the vessel had been reduced to considerably 

 below 001 mm. of mercury. The zinc disk X, whose surface 

 had been carefully scraped before it was inserted in the 

 apparatus, was raised until contact was made between 8 

 and C The polonium plate P was then charged to various 

 positive potentials by means of the storage-battery, and the 

 corresponding currents between P and X were measured 

 with the Dolezalek quadrant electrometer joined to C. The 

 capacity of the quadrants and the attached electrical sj'stem 

 was found to be 140 e.s.u. 



The values of the applied potentials and the currents they 

 produced are given in Table I., and a curve representing 

 them is given in PL VII. fig. 2. 



* Logeman, Proc. Roy. Soc, Series A, vol. lxxviii. Sept. 6, 1907. 



