Mr Toiunsend, Secondary Rontgen Rays. 



225 



Effect of Pressure on Conductivity. 



4. When the primary rays pass through a gas and precautions 

 are taken to exclude surface effects, the number of ions produced 

 per cubic centimetre is proportional to the pressure of the gas. 

 The relation between conductivity and pressure is however more 

 complicated when the surface radiation acts on the gas. In order 

 to obtain a curve showing this relation, some experiments were 

 made on the conductivity of air between two co-axial cylinders. 

 The larger cylinder, 3'2 centimetres in diameter, was of brass, and 

 the rays entered it through an aluminium window. The inside 

 cylinder, 1 centimetre in diameter, was made of very thin aluminium 

 through which the rays could easily pass After traversing the gas 

 between the two cylinders, the rays fell on the surface of the larger 

 cylinder opposite the window and excited a secondary radiation. 

 The electrical arrangements for obtaining the maximum deflections 

 were made in the ordinary way. The outside cylinder was raised 

 to a high potential by connecting it to a terminal of a battery of 

 lead cells, the other terminal being to earth ; the small aluminium 

 cylinder was insulated and put in metallic connection with the 

 insulated quadrants of an electrometer. The results of observations 

 on the maximum deflection when the rays ionize the gas for 

 15 seconds, are shown graphically in curve I. figure 4. The 



ordinates represent the deflections in arbitrary units and the 

 abscissae the pressures in centimetres of mercury. 



In the above experiment the rays passed through a sheet of 

 aluminium ^-millimetre thick before passing through the window 

 in the brass tube. A second series of observations were taken 

 with the brass tube coated inside with a cylinder of aluminium 



