1 74 Scientific lutelligence. — Mechanical Philosophy. 



When a jointed wire, composed of alternate pieces of different 

 metals, having different conducting powers, is made the medium 

 of communication between opposite poles, that which is the slow- 

 est conductor becomes most intensely heated. Thus, if the alter- 

 nate pieces consist of platina and copper, the former may become 

 fully ignited, while the latter exhibit no signs of incandescence. 



In fluids, if their conducting power be diminished by the inter- 

 position of solid particles, the evolution of heat is much greater. 

 This is manifest in causing the current to pass through moisten- 

 ed cotton, or through the fresh stem of a succulent plant. In the 

 latter case, the heat becomes so great as to cause the sap to boil 

 at the points where the platina wires are inserted. It is found 

 also, as might be foreseen, that fluids which evolve the least gas 

 become the most heated. In water, the calorific effect is greater 

 at the positive pole where oxygen is disengaged, than at the ne- 

 gative, where double the volume of hydrogen is set free. 



It is well known that the greatest heating power is obtained when 

 a given surface of metal is employed in the smallest number of plates. 

 A single pair of one foot square each may ignite and melt metallic 

 wires which a pile of eighteen pairs of sixteen square inches each 

 cannot even warm, although the sum of the surfaces is exactly the 

 same, and they are charged with the same quantity of water and 

 acid. It is necessary however in these cases to distinguish between 

 this igniting and melting power and certain other calorific effects, for 

 to produce the combustion of metallic leaves, to evolve light and 

 heat from charcoal points, or to effect an elevation of temperature 

 in liquids traversed by the voltaic current, number is also requisite. 

 Thus a pile of sixty pairs, capable of producing the three last classes 

 of phenomena, cannot redden the finest wire of platina or iron ; 

 while ten pairs of the same pile produce the latter effect, but cannot 

 determine the former. 



The different calorific effects of the pile must not therefore be 

 confounded with each other, but should be classed, simply, according 

 to the conductors necessary to their production. If the conductor be 

 perfect, continuous and homogeneous, as a metallic wire, the effect, 

 whether calorific or magnetic, will be the more intense, the smaller 

 the number of elements under a given surface. If the conductor 

 be imperfect, or discontinuous, as in charcoal points or metallic leaves, 

 or if it be heterogeneous as in the case of metallic plates immersed in 

 a liquid interposed between them, then the given surface must be 



