214 Mr. Prideaux on the Theory of Thermo-electricity. 



in favour of the conducting power of copper, down to a very 

 low heat, and at \ an inch distance. 



15. To try at still shorter distances, two other lengths of 

 the same wire were twisted together for \ an inch, then separated 

 for an inch, and the other ends brought near together, for 

 holding in a pair of cold pliers. (The twist was tarnished, to 

 prevent metallic contact between the mercury and copper, 

 which also was the object of the thread-binding in the former 

 part of the experiment.) Spots of lard were now placed, at 

 equal distances from the twist, on the iron and on the copper, 

 and the twist plunged into the warm mercury. The lard became 

 transparent (by melting) first on the copper: and this on repe- 

 tition, whatever the distances, down to ^th of an inch; and what- 

 ever the temperature, so long as it was sufficient to melt the 

 lard at all. Varying the sizes and proportions of the wires, 

 tinning their surfaces, to prevent unequal waste of heat, in 

 transitu, by radiation, did not alter the result. In every in- 

 stance, for equal distances, the lard first became transparent 

 upon the copper wire; which is thereby proved, for low heats 

 and short distances, to retain its character as the better con- 

 ductor for caloric. 



16. To ascertain the conducting powers for electricity of 

 very low tension, a pair of iron and a pair of copper wires, 

 atomically proportioned as before, and each 2 inches long, 

 had \ an inch at each end bent down at right angles, and the 

 points tinned, for equal contact with the mercury. Through 

 these wires, two other mercury boxes were connected with 

 those of the magnetest, an iron and a copper wire being em- 

 ployed on each side. Into the further boxes dipped the ends 

 of a thermo-electric pair, of copper and tinned iron, of which 

 the other ends were twisted together, and the twist was ex- 

 posed to an equable heat, until the needle became stationary 

 at 18°. 



The short iron wires were now removed, leaving the con- 

 duction to be carried on* by the short copper ones only: no 

 appreciable change took place in the deflection. 



The iron wires being now replaced, the copper ones were 

 removed, leaving the conduction entirely to the iron. The 

 needle receded more than a degree, and sometimes nearly 2°, 

 confirming the superiority of copper as an electrical con- 

 ductor, for extreme low tension and very short distances. 



17. But as heat also enters into the question, the apparatus 

 remaining as described, a spirit-lamp was brought under the 

 middle of the copper leg of the thermo-electric pair, and con- 

 tinued there until the point acted upon by the flame became 

 red hot. Little or no change took place in the deflection. 



