364 Measuring Conductivity along a Glow Discharge. 



yielded by the latter method are in good agreement with the 

 measurements of the electric force at various parts in the 

 stria? made by Sir J. J. Thomson with the aid of a pencil of 

 cathode rays sent at right angles across the axis of the 

 striae *. 



Here the maximum electric force was shown to occur at 

 the bright parts of the striations very near their negative 

 end. Now, if at the pressures used the velocity of the 

 ions is assumed to be proportional to the electric force at 

 the spot, a minimum number of ions and therefore the mini- 

 rautn conductivity can be expected to occur at a maximum 

 of electric force f , hence the agreement. 



In conclusion a number of very preliminary experiments 

 were made with flames. They were simply brought in the 

 space between the ends of the Lecher wires, and the reduc- 

 tion of the wave-amplitude thus obtained again afforded an 

 indication of the conductivity of different flames or of dif- 

 ferent parts of the same flame. In the electrodeless method 

 the uncertainties arising otherwise from the polarization at 

 the electrodes and from their electron emission if hot are 

 obviously again eliminated. 



The following are a few of the results obtained : — 



When the unabsorbed wave-amplitude corresponded to 

 49 centimetres thermogalvanometer deflexion, the latter was 

 reduced to 43 when the flame of an ordinary Bunsen burner 

 was brought between the wires. If thereupon a small piece 

 of asbestos soaked in a concentrated NaCl solution was 

 brought near the bottom part of the Bunsen flame, a con- 

 siderably greater absorption took place and the galvanometer 

 deflexion fell to 29. 



Again, when some caesium -aluminium-sulphate was brought 

 in the pure Bunsen flame the deflexion decreased from 49 to 

 26. This is another proof that the increased conductivity. of 

 salt-laden flames, as measured with test-electrodes, is not 

 wholly due to the presence of the electrodes, as is sometimes 

 supposed, but it is a peculiarity inherent in flames. 



In conclusion, I wish to thank Sir J. J. Thomson for his 

 valuable advice and kindness in putting at my disposal the 

 instruments of the laboratory. 



Cavendish Laboratory, 

 Cambridge. 



* Phil. Mag. xviii. p. 441 (1909). 



t J. J. Thomson, Cond. of Electr. thr. Gases (2nd Ed.) p. 561. 



