366 



Prof. J. J. Thomson on easily Absorbed 



could be moved from one position to another. The tube 

 with the aluminium windows was kept fixed between the 

 fixed anode and the moveable cathode; when the cathode was 

 moved the windows were thus brought into different positions 

 in the negative glow. The potential-difference between the 

 cathode and anode was measured by a Kelvin vertical volt- 

 meter, the current through the tube by a D'Arsonval gal- 

 vanometer ; this current was adjusted by a liquid resistance 



Fig. 3. 



( 



to Electrometer 



cathode 



placed in series Avith the tube. The pressure in the tube was 

 measured by keeping the tube connected with a mercury- 

 pump to which a McLeod gauge was attached. The follow- 

 ing (p. 367) is a specimen of the observations made with this 

 apparatus, showing the way in which the intensity of the 

 radiation produced by the impact of the cathode particles 

 against the windows varies with the distance of the windows 

 from the cathode. 



The relation between the rate of leak and the distance 

 from the window is shown graphically in fig, 4. The rate of 



