110 Mr. J. Brown on the Theory of Voltaic Action. 



negative on the side towards the copper. In order to prove 

 this, a disk of thin vulcanite, with a hole 

 in its centre and a radial slit, c d, had two 

 paper segments, a and 5, fixed on it. These 

 were moistened with water, and the whole 

 placed in the apparatus described before in 

 the same position as one of the bimetallic 

 rings. The needle was set over the slit c d; 

 and each of the points a and b of the paper 

 segments was in connexion with a slip of 

 moist blotting-paper which passed out of the case. The outer 

 ends of these slips lav. side by side on a plate of vulcanite. 

 Touching either one of them with a piece of zinc or copper, or 

 with one end of an insulated copper-zinc pair soldered together, 

 had no appreciable effect on the electrified needle ; but when 

 the zinc end of the pair was placed on one slip and the copper 

 end on the other, the index, light-spot at once deflected about 

 10 centims., showing the paper in connexion with zinc posi- 

 tive. When a copper-iron pair was used the iron side was 

 positive ; but on placing a drop of potassium sulphide at the 

 junction of the copper and damp paper, the copper side became 

 positive. If the copper-iron pair, instead of being soldered 

 together was joined by a drop of water, no deflection occurred, 

 or only a very small one ; but the addition to the connecting 

 water-drop of a small quantity of potassium sulphide caused 

 a vigorous deflection of the needle, the segment in connexion 

 with the copper being now negative on account of the current 

 flowing across the connecting drop from copper to iron. 



The slit c d corresponds to the dividing plate in the electro- 

 lyte ; and if we suppose this plate to be air and its thickness 

 to be increased, replacing the liquid till nothing but a mere 

 film remains on each metal, we have then the conditions of 

 Volta's condenser experiment, where, the plates being close in 

 front of one another and in metallic connexion, the film of 

 condensed moisture &c. on the zinc plate has on its outer sur- 

 face a positive charge, that on the copper a negative, the layer 

 of air between them preventing the combination of the two 

 electricities. It may be urged that the better the plates of a 

 condenser are ground or fitted together, the more apparent is 

 the contact effect ; but it is scarcely to be supposed that we 

 have yet had any experiments with plates so well surfaced that 

 it was certain no air layer was present between them, or so 

 well mounted that they could be kept exactly parallel while 

 being separated, all points on their surfaces separating at the 

 same instant. 



In my former paper, the production of a difference of elec- 



