404 



Mr Rutherford, Tlie Discharge of 



[Feb. 21, 



placed at a distance of 10 cm. from a neighbouring plate gave a 

 rate of leak 12 times as fast as under the action of the ultra- 

 violet light alone. The blast seems to assist the electromotive 

 force in removing the negative charge. The presence of this 

 action must be taken into account in all cases where currents of 

 air impinge on negatively electrified surfaces. 



The following figure (Fig. 1) shows the general arrangement 



Fig. l. 



of the experiment to show the effect of blowing a current of air by 

 a negatively electrified plate on which ultra-violet light is allowed 

 to fall. 



A blast of air from a bellows or a gasometer passed between 

 two parallel plane electrodes B and C, and then through an in- 

 sulated aluminium cylinder T. The air before reaching the plates 

 was free from dust as far as possible by passing through a bulb G 

 tightly packed with cotton -wool. An arc light A was used as a 

 source of ultra-violet light. The light after passing through a 

 quartz plate Qj which covered an opening in a metal screen LM 

 surrounding the arc, passed through a second quartz plate Q 2 , 

 through the fine wire-gauze B and then impinged on the metal 

 plate G, which was generally of polished or amalgamated zinc. 

 The broken lines show the position of insulators by which the 

 gauze B, the plate G, and the tube T were all insulated from one 

 another. An insulated wire W passed centrally down the 

 tube T. 



In a particular experiment the plate G was 4*2 cms. long, 

 1*5 cms. wide and - 8 cm. from the gauze B. Since the area of the 

 rectangular orifice through which the air passed was only 1'2 sq. 

 cms., velocities of the air of 300 or 400 cms. per sec. could be 

 readily obtained. 



