34 Mr. J. A. McClelland on the Conductivity of 



we raise the potential of B the leak from C diminishes until, 

 when the potential of B is high enough to give the maximum 

 leak from it, we get no leakage from C. All the carriers 

 have been discharged before they pass B, and the gas above 

 it possesses no conductivity. The maximum rate of leak 

 from B is the same whether it is charged positively or nega- 

 tively, showing that equal amounts of positive and negative 

 electricity are carried by the ions. 



We can calculate the amount of electricity on the carriers 

 of one sign per unit volume of the gas from the flame. A 

 terminal placed in the axis of a cylinder through which the 

 gas was moving and joined to a capacity (including that of 

 the quadrants of the electrometer) of 280 C.G.S. units fell in 

 potential at the rate of 5 volts per second, which corresponds 

 to j electrostatic units of charge per second. We know that 

 the gas in a cross-section of 3 sq. cm. around the terminal 

 was discharged, and the velocity of the stream of gas past the 

 terminal was 130 cm. per second, so that the charge per unit 

 volume was ^ 3 electrostatic unit. If each carrier has the 

 atomic charge, the number in unit volume is of the order 

 10 9 . Or the ratio of the number of charged carriers to the 

 number of molecules is of the order 10 -12 . These numbers 

 refer to the conducting gas at a point about 17 cm. from the 

 flame ; closer to the flame the number of dissociated atoms is, 

 of course, much greater, and the number falls away rapidly 

 as the gas moves away further from the flame. 



2. Recombination of the Ions. 



We can measure the rate at which the positive and nega- 

 tive ions recombine by determining the conductivity of the 

 gas at different times after it is drawn away from the flame. 

 To do this, the gas from the flame passes up a cylindrical 

 tube which is connected to earth, and an insulated metal rod 

 is lowered into the tube from the top and placed in its axis. 

 This rod is connected to a pair of quadrants of an electrometer, 

 and charged to a potential high enough to ensure that all the 

 carriers are discharged in a short distance after reaching the 

 lower end of the rod. By lowering the rod further and 

 further into the tube, and measuring the rate of leak in each 

 position, we get the conductivity at different distances from 

 the flame. By measuring the temperature at different points 

 of the tube, and therefore knowing the relative velocities of 

 the current of gas at these points, we can express the con- 

 ductivity of the gas in terms of the time since it left the 

 flame. The following table shows the result of an experi- 

 ment : — 



