194 



Mr. E. B. Rosa on the Specific 



4. Variation of Conductivity. — The resistance of different 

 samples of distilled water is very different, and yet the 

 difference noted in the force of attraction between the 

 electrodes was relatively small. A single drop of a dilute 

 solution of copper sulphate was added to a half litre of 

 distilled water ; the conductivity was increased about ten 

 times while the force of attraction diminished about 15 per 

 cent. Hydrant water, of comparatively large conductivity, 

 showed a force 10 per cent, to 40 per cent, less than distilled, 

 according to the potential, the effect of polarization being 

 relatively greater at low potentials. 



Thus, as far as the force of attraction depends upon the 

 conductivity, it seems to be less as the conductivity is greater, 

 and that because of polarization. Could the latter be 

 eliminated in the case of these impure waters, the force would 

 probably be practicably independent of the conductivity. 

 Polarization reduces the force of attraction, as it reduces the 

 current, in two distinct ways. First, by the counter-electro- 

 motive force excited which diminishes the effective fall of 

 potential, and, second, by increasing the resistance at the 

 electrodes. This decreases the current by increasing the 

 total resistance ; but it decreases the force not at all because 

 of an increase in the total resistance, but because the addi- 

 tional resistance is introduced at one or two places and not 

 uniformly throughout the liquid. The result is that the fall 

 of potential is less in the liquid near the needle than before, 

 and therefore the force on the needle is less. 



The effect of introducing between the electrodes a stratum 

 having a high resistance is strikingly shown in the following 

 experiments : — 



A. — A plate of mica (in, fig. 3, which is a horizontal 

 section of the electrolytic cell) was inserted between the needle 

 n and the plate p, and parallel to the latter. The following 

 observations were made, the plate being set at distances from 

 the needle given in column 3, the mica being in each case 

 about 1*5 millim. from the platinum plate : — 



Table VII.— Water. 



No. 



Potentials. 



Distances. 



Deflexions. 





1. 



40 volts 



1-0 cm. 



4-1 50 cm. 



Without mica plate. 



2 



•±0 „ 



1-0 „ 4- 0-84 „ 



With mica plate. 



3. 



40 „ 



0-55 ,. 0-00 „ 





4. 



40 „ 



03 „ - 068 „ 



„ 



