1906.] 



on Electric Discharge and Spectroscopy. 



197 



magnet, so aiTanged that its lines of magnetic force in the discliarge 

 tube are along the Hne joining the cathode and the anode. The effect 

 of the magnetic field is to make the corpuscles move along the lines of 

 force, and thus, without altering the number of corpuscles emitted by 

 the cathode, it concentrates their paths, and so increases the maximum 

 current density in the tube. When the magnet is on, ionization by 

 colhsion and luminosity both occur at a much lower potential differ- 

 ence than when it is off, and it is easy to arrange matters so that 

 keeping the potential difference constant, the discharge is luminous 

 when the magnet is on and dark when it is off. When the potential 

 difference is too small to produce a bright discharge even when the 



1100 



Discharge 



^n JVauSwHeAT Disch arg^ 



° fj£iD^° ^^ ^o aoVolts 



Fig. 3. 



magnet is on, the current through the tube is often greater when the 

 magnet is on than when it is off. By placing the magnet so that the 

 lines of magnetic force are across the line joining the cathode to the 

 anode, we can render the paths of the corpuscles more diffuse than they 

 would be without the field, so that the maximum current density is less 

 when the magnet is on than when it is off. In this case it requires a 

 larger potential difference to produce a luminous discharge with the 

 magnet on than with it off. Similar effects produced by a magnet on 

 another kind of discharge are described in my " Recent Researches," 

 page 105. 



The potential difference P, just when the glow commences, when 



