1909] on the Electrical Properties of Flame. 467 



negative electrode the current must be carried entirely hj positive 

 ions moving- towards it, and at the positive electrode the current must 

 be entirely carried by negative ions. We find that the resistance 

 near the negative electrode is much greater than near the positive 

 electrode, so that we conclude that the negative ions carry the current 

 more easily than the positive ions. With a given electric force, the 

 negative ions move very much faster than the positive ions. It has 

 been shown experimentally that the velocity of the negative ions is 

 about 10,000 cm. per sec. for one volt per cm., Avhile that of the posi- 

 tive ions is about 100 times smaller than this. 



In the flame away from the electrodes the electric force is found 

 to be proportional to the current, so that here the flame obeys Ohm's 

 law like a metallic conductor. Its conductivity is about 10'' times less 

 than that of copper. In the equation Y = M- + B^/, the term Br// 

 is the part of the E.M.F. used up between the electrodes, so it is 

 proportional to the current and to the distance. Professor Sir J. J. 

 Thomson has shown theoretically that the drop of potential near the 

 electrodes should be proportional to the square of the current, as is 

 found experimentally to be the case. 



The conductivity of a Bunsen flame may be compared with the 

 conductivity of liquids, such as water. In pure water some of the 

 molecules are dissociated into ions and the water is a conductor, 

 although only a poor one. But if a salt like sodium chloride is 

 dissolved in the water the salt dissociates into ions almost completely. 

 and the conductivity is greatly increased. Suppose we hold a bead of 

 salt on a platinum wire in a flame, then the salt volatilises and the 

 flame is filled with its vapour, and, just as with the water, the conduc- 

 tivity is enormously increased. 



With the long flame and an electrode at each end, we can try the 

 effect on the current of putting salt in different parts of the flame 

 between the electrodes. In this way it is easy to show that the current 

 is practically unchanged, unless the salt vapour is put in close to the 

 negative electrode, but in that case it produces a very great increase 

 in the current. This confirms the conclusion that nearly all the 

 resistance to the passage of the current is situated close to the negative 

 electrode. When the salt is put in anywhere it diminishes the resist- 

 ance there to a small fraction of its value, but it is only close to the 

 negative electrode that the diminution in the total resistance is 

 appreciable. If we measure the potential difference between two points 

 in the flame away from the electrodes, and then put salt vapour in the 

 flame between them, we find that the P.D. drops to a small fraction 

 of its value although the current is the same as before. This shows 

 clearly that the salt vapour greatly increases the conductivity where- 

 ever it is put in. 



If some salt is put on the negative electrode, the sudden drop in 

 potential there almost disappears, and we get a nearly uniform poten- 

 tial gradient from one electrode to the other, so that now the resist- 



