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Mr. J. A. McClelland. 



That the cathode rays do carry a negative charge has been con- 

 clusively shown by Perrin,* and the method here used for testing 

 the charge carried by the Lenard rays is the same in principle as that 

 used by him on the cathode rays. 



A tube as shown in fig. 1 was constructed. 



A is the anode and is connected to earth, C is the cathode. 



Inside the tube are fixed two short metal cylinders a and h The 

 inner cylinder b has a small circular opening at d, facing towards the 

 cathode, and is held in position by a stiff wire which passes through 

 the glass at E, and is connected to a pair of quadrants of an electro- 

 meter. 



If then the cathode rays entering the cylinder b carry a charge 

 the electrometer detects it, even though the rays may not give up 

 their charge to the metal walls of the cylinder. 



The outer cylinder a, which has also a small opening at c facing 

 the cathode, is kept connected to earth by means of a wire passing 

 through the glass at D, and thus protects the inner cylinder from 

 -outside electrical action. 



S is a screen (shown in front view at Si), made of a circular disk 

 of lead with the central part cut away, and the hole covered with a 

 thin piece, /, of aluminium about O'Ol mm. thick. 



The screen is mounted on a piece of iron, h, to enable it to be 

 moved about in the tube by means of a magnet. The screen is of 

 such a diameter as to almost touch the glass all round, just allowing 

 freedom of motion under the action of the magnet. 



* * Nature,' January 30, 1896. 



