carried by the a and (3 Rays of Radium. 197 



aluminium-foil corresponded to about 0*19 milligram of 

 radium bromide. It was thought possible that some of the 

 radium might be absorbed in the pores of the aluminium 

 plate, and it was for this reason that a set of experiments was 

 made with a glass plate. The agreement of the results, 

 however, obtained with the two plates indicate that such an 

 effect, if it exists, is small. 



One of these radium-coated plates was placed in the 

 vacuum-vessel D in the position of the plate A. 



With the view of reducing as far as possible the number of 

 electrons emitted by the impact of the a particles on the 

 absorbing plate, the a particles were shot into a rectangular 

 copper vessel BC, the bottom of which was covered with a 

 layer of aluminium-foil '00034 cm. in thickness. Most of the 

 a, particles passed through this aluminium -foil, and were 

 stopped by the metal walls of the vessel. 



The brass vessel D was exhausted by means of a mercury 

 pump to a very low pressure. In later experiments, it was 

 found that a low vacuum could be more easily produced by 

 Dewar's method of absorption of the residual gas by charcoal. 

 A side tube, containing cocoanut charcoal immersed in liquid 

 air, was connected to the vessel D. The air was first 

 partially exhausted by means of a mercury pump, the cocoa- 

 nut charcoal being heated to drive off the absorbed gases. 

 This method was found to be very convenient to keep a good 

 vacuum without the necessity of continuous pumping. The 

 apparatus was placed between the pole-pieces of a large 

 electro-magnet, so that the field was parallel to the plane of the 

 plates AB. The following table gives the numbers obtained 

 for the current, when the lower plate was charged positively 

 or negatively, and when the magnetic field was on or off : — 



Potential of lower 

 plate. 



Current in ar 



Ditrary units. 



Without magnetic 

 field. 



With magnetic 

 field. 









4-36 



+ 2 volts. 

 _ 2 



-|- 20 

 - 2-5 



+•33 j oJ,) 



+ 4 ., 

 -4 „ 



-f 2-8 

 -3-5 



tin 



tl :; 



+ 34 

 - 4-0 



+|? }-435 



+ 84 

 -84 



+ 3-5 

 - o-2 



i'llr 05 



The aboA'e numbers refer to the radium-coated glass plate. 



