Magnus on Thermo-electric Currents. 83 



was sufficient to give rise to a current. When a portion of a 

 wire which had been rendered hard by drawing was heated to 

 redness and thus softened, on warming the point of junction 

 between hard and soft, a cm-rent was always obtained. In like 

 manner, when a portion of the wire was rendered harder by ham- 

 mering, a current was produced on heating the junction of hard 

 and soft. 



For these experiments a particular arrangement of apparatus 

 was devised 5 and to prevent any new change in the structure of 

 the wire, it was rarely heated beyond the temperature of boiling 

 water. Particular care was also taken to preserve the points 

 where the two ends of the wire experimented with joined the 

 wire of the galvanometer at the same temperature, a condition 

 absolutely necessary to prevent the formation of a current at 

 these points of junction. 



M. Becquerel was the first to demonstrate, that when a wire 

 is knotted and heated in the vicinity of the knot, a current is 

 exhibited. As, however, M. Becquerel employed a red heat in 

 his experiments, it is possible that the current obtained was due 

 to a softening of a portion of the wire, while the knotted portion 

 retained its hardness. Such a result is still more probable in 

 the case where the point of junction of a thick and thin wire is 

 strongly heated. Up to the present time it has been an accepted 

 fact, that a difference in point of thickness merely is sufficient 

 to originate a current. 



For the stricter examination of this question two semi-cylin- 

 ders of brass were procured, and along the axis of each a semi- 

 cylindrical hollow was worked out from end to end. Into this 

 hollow a brass wire was accurately fitted ; so that when one piece 

 was placed upon the other, the whole had the appearance repre- 

 sented in Plate III. tig. 1. The slightest heating of the wire in 

 the neighbourhood of its thick case was sufficient to develope a 

 current, and the currents developed when the wire was heated 

 at both ends of the case in succession were in opposite direc- 

 tions. 



What is the proximate source of the electricity in this case ? 

 is it the result of a mere difference in point of thickness; is it 

 to he referred to a difference in chemical composition ; or is it 

 due bo :i difference of hardness between the wire and its encom- 

 ring cylinder ? If a piece of metal be laid upon another piece 

 of a different metal, in the manner represented in fig. 2, when 

 the point c is warmed a thermo-current is evoked, which circu- 

 lates for the most part within the boundaries of the two pieces. 

 If, however, the extreme ends of the bar be united with a galva- 

 nometer, a branch current will exhibit itself; hence if the thin 

 wire spoken of above and its encompassing sheath be not per* 



G2 



