CONVECTION, OR CARRYING DISCHARGE — CURRENTS. 151 



If a drop of mercury be hung from a ball used to raise a column of the fluid (1598.), 

 then the shape of the drop seems to show currents travelling in the fluid in the di- 

 rection indicated by the arrows (fig. 27.). 



1600. A very remarkable eff'ect is produced on these phenomena, connected with 

 positive and negative charge and discharge, namely, that a ball charged positively 

 raises a much higher and larger column of the oil of turpentine than when charged 

 negatively. There can be no doubt that this is connected with the difference of 

 positive and negative action already spoken of (1480. 1525.), and tends much to 

 strengthen the idea that such difference is referable to the particles of the dielectric 

 rather than to the charged conductors, and is dependent upon the mode in which 

 these particles polarize (1503. 1523.). 



1601. Whenever currents travel in insulating dielectrics they really efl^ect dis- 

 charge ; and it is important to observe, though a very natural result, that it is indif- 

 ferent which way the current or particles travel, as with reversed direction their state 

 is reversed. The change is easily made, either in air or oil of turpentine, between 

 two opposed and related rods, for an insulated ball being placed in connexion with 

 either rod and brought near its extremity, will cause the current to set towards it 

 from the opposite end. 



1602. The two currents often occur at once, as when both terminations present 

 brushes, and frequently when they exhibit the glow (1531.). In such cases, the 

 charged particles, or many of them, meet and mutually discharge each other (1548. 

 1612.). If a smoking wax taper be held at the end of an insulating rod towards a 

 charged prime conductor, it will very often happen that two currents will form, and 

 be rendered visible by its vapour, one passing as a fine filament of smoky particles 

 directly to the charged conductor, and the other passing as directly from the same 

 taper wick outwards, and from the conductor ; the principles of inductric action and 

 charge, which were referred to in considering the relation of a carrier ball and a con- 

 ductor (1566.), being here also called into play. 



1603. The general analogy and, I think I may say, identity of action found to exist 

 as to insulation and conduction (1338. 1561.) when bodies, the best and the worst in 

 the classes of insulators or conductors, were compared, led me to believe that the 

 phenomena of convection in badly conducting media were not without their parallel 

 amongst the best conductors, such even as the metals. Upon consideration, the cones 

 produced by Davy* in fluid metals, as mercury and tin, seemed to be cases in point, 

 and probably also the elongation of the metallic medium through which a current of 

 electricity was passing, described by Ampere-J- ; for it is not difficult to conceive, 

 * Philosoplucal Transactions, 1823, p. loS. f Bibliotheque Universelle, xxi. 47. 



