150 On Surface-resistance at Electrodes. 



largely the amount of " surface-resistance/'' and the strength 

 of current, in a thermoelectric couple composed of a metal 

 and an electrolyte, we may infer that it would also, chiefly 

 by altering that " resistance/'' affect the strength of current 

 in a voltaic cell. 



In order to experimentally examine this, the lower platinum 

 plate in the thermoelectric apparatus, already described, was 

 replaced by a rod of amalgamated zinc 8'0 mm. long and 

 2*0 mm. thick ; the upper platinum plate being retained in 

 its place. The tube was then filled with the cold mixture of 

 one volume of sulphuric acid and ten of water. 



With the entire apparatus and liquid at 14° C, the electro- 

 motive force (determined by the above-named method) was 

 •704 volt, and the strength of current without additional 

 resistance in the circuit was *022 ampere. 



With the platinum plate and upper portion only of the 

 liquid at 95° C, and the zinc electrode and liquid around it 

 at 14° C, the electromotive force was "763 volt, and the 

 strength of current '0705 ampere. 



These results show that the electromotive force was in- 

 creased about 8*38 per cent., and the strength of current 220 

 per cent., or to 3'2 times its original amount, by raising 

 the temperature of the platinum negative plate 81 C. degrees. 

 This increase of strength of current was about 24 per cent, 

 greater than when the lower plate was composed of platinum. 

 This great increase of current was, I consider, largely due to 

 decrease of " surface-resistance " at the negative plate, and 

 in some measure also to the increase of electromotive force 

 attending increased thermoelectro-positive potential of the 

 liquid ; also to several other manifest but minor circumstances, 

 such as diminished conduction-resistance of the liquid, &c. 



Third. — With regard to the thermoelectric phenomena of 

 metals. Some experiments were made to try and detect any 

 special degree of resistance at the junctions of ordinary 

 metallic thermoelectric couples, in the following manner : — 



The first couple .consisted of a smooth horizontal bar of 

 bismuth, and one of antimony, soldered end to end ; each bar 

 being 2*0 cm. long, '7 cm. wide, and *5 cm. thick. Whilst a 

 current of '2 ampere was being passed from one end of the 

 bar to the other, the ends of two thin copper wires, about 

 1*0 cm. apart, forming the terminals of a Thomson's reflecting- 

 galvanometer of 3040 ohms resistance, were slid very slowly 

 at right angles to and in a horizontal direction, from one end 

 to the other, to and fro, along the upper surface of the couple ; 

 the direction of voltaic current being occasionally reversed. 

 Under these conditions, a small portion of the current passed 



