992 Sir J. J. Thomson on a 



The light and dark parts of the Discharge. 



At the places where there is ionization there will he 

 recombination, and where there is recombination we should 

 expect to*find luminosit}*-. Now, when E is > E there will 

 be some ionization, so that we should expect the parts o£ the 

 tube where E>E to be luminous. If the gas is one which 

 has a radiation potential lower than the ionizing potential, 

 then luminosity may set in before ionization, and all parts 

 of the tube corresponding to portions of the graph above 

 the line E = R, when B,=V k e, V* being the radiation 

 potential, will be luminous ; the other portions will be dark. 

 The source of the light when E is between R and E will 

 be atoms which have not been ionized and so have not been 

 charged, and have therefore not experienced any tendency 

 to move one way or another in the tube. At the places 

 w T here E is greater than E there is ionization ; the atoms 

 which have recombined and which are the sources of the 

 light have been positively charged, and under the electric 

 forces in the tube will have moved towards the cathode. 

 "We may get some idea of the velocity of the positive ions 

 by supposing that at the pressure in the tube the mobility 

 of the positive ion is inversely proportional to the pressure. 

 If X is the electric force and h the mobility, the velocity of 

 the positive ion is JcX. Now the force along the positive 

 column is approximately proportional to the pressure ; thus 

 X is directly and k inversely proportional to the pressure, 

 so that hX. will be approximately independent of the 

 pressure. At atmospheric pressure X for air is about 

 3(3,000 volts per centimetre, while the mobility of the 

 positive ion in air is such that its velocity is about 

 1*5 cm./sec. for the force of a volt per centimetre. Thus 

 the velocity of drift of the positive particles towards the 

 cathode will be about 45,000 cm./sec. This is of the order of 

 the velocity of sound through the gas, so that the particles 

 which are to give out the light are moving towards the 

 cathode with something like the velocity of sound. They 

 will not get far, however, before they get neutralized, when 

 they will soon lose their velocity. 



The drift of the positive ions before they begin to emit 

 light will make the curve which represents the distribution 

 of luminosity along the tube differ appreciably from that 

 representing the distribution of the energy of the electrons, 

 and will make the places of maximum luminosity nearer 

 to the cathode than the places of maximum energy and 

 ionization. The places where the ions are produced are 

 the places where the energy and speed of the electrons are 



