Phenomena of the Capillary Electroscope. 



101 



Exp. No. 27) ; and further, by the surfaces of the meniscus not 

 becoming oxidised when made the positive pole in either of the 

 various solutions usually employed ; provided in all these cases that 

 ordinary chemical action is absent and that the current is sufficiently 

 weak. 



The reverse directions of movement produced by the same direction 

 of current in dilute sulphuric acid and in solution of potassic cyanide, 

 probably cannot both be explained by the theory that the movements 

 are due to electro-chemical exidation and deoxidation. The upward 

 movement caused by a downward current can hardly be due to electro- 

 lysis, because that action, by continually destroying the outer film of 

 mercury, tends to diminish its surface tension. 



Being largely dependent upon electric conduction, the amount of 

 movement of the mercury is affected by every circumstance which 

 alters the conductivity. The part of the circuit which offers the 

 greatest amount of conduction resistance is the slender column of 

 solution between the meniscus and the end of the capillary tube ; 

 there the resistance is considerable. The nearer, therefore, the 

 meniscus is to the end of the tube, and the shorter the column of 

 solution (as well as that of the mercury), the more rapid is the move- 

 ment, especially if the tube is slightly larger in diameter towards that 

 end ; this circumstance also tends to make the downward movement 

 an accelerated one, even in a tube of uniform diameter, and the 

 upward movement the reverse ; and also accounts for the fact that the 

 instrument is more sensitive to a downward current than to an up- 

 ward one, unless the tube becomes narrower downwards at too rapid a 

 rate. 



The secondary current of an induction coil was not suitable for 

 working the instrument, because it produced electrolysis. On many 

 occasions an electrophorus was used as the source of electricity, and 

 this also produced a similar effect. On charging the electroscope 

 with it, either by induction or by contact, the movements were freely 

 produced, provided the electroscope was not insulated. If it was 

 perfectly insulated the movements did not occur, and if it was 

 imperfectly insulated and then charged by momentary contact, the 

 mercury continued to run out at the end of the capillary tube after 

 removal of the electrophorus, and ceased to flow by discharging the 

 instrument. 



I made a capillary tube in accordance with the annexed sketch, 

 with a pressure tube A, and a platinum wire electrode Pt ; and filled 



Tig. 3. 



