Contact with Dielectrics. 485 



writer suggested that the salt vapour was only ionized when 

 in contact with the hot platinum electrodes. This view 

 enabled a great many experimental results to be simply ex- 

 plained, but it appears now that the true explanation of these 

 results is the smallness of the potential gradient in the flame, 

 except close to the electrodes. It was supposed previously 

 that when the P.D. used was large, so that the current only 

 increased slowly with increasing P.B., that then the current 

 was nearly at its maximum possible value, and so was a measure 

 of the number of ions formed between the electrodes. It is 

 clear now that this supposition was incorrect, and that the 

 current is really always very far from its saturation value. 

 It is of course probable that some surface ionization on the 

 hot electrodes does occur, but, at any rate, when the elec- 

 trodes are not very near together, it does not appear to be 

 sufficient to appreciably influence the conductivity of the 

 flame. 



The view that the salt vapour is ionized throughout the 

 flame was adopted by Arrhenius (Wied. Ann. xlii. p. 18, 

 1891), E. Marx (Ann. de Phys. ii. pp. 768, 798, 1900), and 

 by F. L. Tufts and Stark in the papers referred to above. 



In conclusion, I wish to say that my best thanks are due 

 to Prof. J. J. Thomson for valuable advice and kind interest 

 shown during the carrying out of these experiments. 



LYI. Contact with Dielectrics. By Kollo Appleyard *. 



Objects. 

 To examine : — 



(1) Whether tinfoil electrodes, pressed against a sheet of dielectric 

 by india-rubber disks, enable accurate determinations of dielectric- 

 resistance to he made. 



(2) The effect upon dielectric-resistance of change of load on such tin- 

 foil electrodes, in the case of press-spahn. 



(3) The effect upon dielectric-resistance of increase or decrease of 

 voltage in the case of press-spahn between tinfoil electrodes. 



(4) The rate and direction of the change of deflexion in direct-deflexion 

 tests on press-spahn, and to determine in how far these changes result 

 from surface conditions, and in how far from internal stresses. 



(5) The effect of reversals of voltage upon dielectric-resistance. 



(6) The effect of prolonged " electrification.'' 



(7) To indicate the probable limits of accuracy with mercury electrodes. 



(8) To point out that Price's guard-wire can be used in sheet tests to 

 eliminate leakage over the sheet surface, as well as over the instruments. 



(9) The retentive force between electrodes and dielectrics. 



* Communicated by the Physical Society : read June 16, 1905. 



