[MCLENNAN- ON THE DELTA RAYS EMITTED BY ZINC 221 



found] 



cms in diameter and about 60 cms in length. This tube carried at its 

 upper end a tap windlass W and at its lower end it was provided with 

 a ground joint for fitting it into the glass heating chamber shown in 

 the diagram. The tube was lined with a thin walled brass tube, which 

 was kept joined to earth through a connection at E. B and D were 

 two guiding rods of brass attached to the inner lining brass tube 

 and M N was a strip of brass which was supported by a cord from the 

 windlass W and had loops on its ends about the guiding rods B and D. 

 An insulated brass rod H was rigidly attached to M N through the 

 intermediary of a short cylinder of amber A. It carried at its lower end 

 a small plate of zinc X with a projecting piece S which came into con- 

 tact with the cup C when the rod H was raised by the windlass. A 

 slender brass rod connected the cup C to a sensitive electrometer. P 

 was a circular plate of copper 2 cms in diameter with a deposit of 

 polonium on its anterior face. As shown in the figure it could be 

 connected as desired to either terminal of a battery of small storage 

 cells. The tube B' was filled with coconut charcoal which was used 

 for the purpose of improving the vacuum made with a Gaede rotary 

 mercury pump. F was a small fused quartz furnace tube and was 

 provided with platinum heating coils as shown. It was held in an 

 upright position by means of a short glass rod sealed into the base 

 of the heating chamber. When making zinc deposits on the surface 

 of the zinc plate X the apparatus was first of all evacuated as highly 

 as possible with the Gaede pump in 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 arrangement 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 C. Under these conditions the zinc plate X was directly in 

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

 ment 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 experi- 

 ments were carried out similar to those described by Logeman^ in 

 his paper on the emission of electrons from metals bombarded by alpha 



1 Logeman. Proc. Roy. Soc. Series A, Vol. 78, Sept. 6, 1907. 



