54 



each case the thermo-electric quality of soft iron intermediate between 

 them. 



These various results show that the character of the effect in each 

 case is decided by distorting stress or by distortion, and leave entirely 

 open, and only to be answered by further experiments, the questions : 

 what is the thermo-electric effect of pressure or traction, applied 

 uniformly in all directions to a metal? and what is the thermo- 

 electric effect of a permanent condensation or dilatation remaining in 

 the metal, when freed from the force by which that condensation or 

 dilatation was produced ? 



Experiments are also described, by which the author found that 

 in soft iron under magnetic force, and in that retaining magnetism, 

 when removed from the magnetizing force, directions along the lines 

 of magnetization deviate thermo-electrically towards antimony ; and 

 that directions perpendicularly across the lines of magnetization in 

 soft iron, deviate towards bismuth, from the unmagnetized metal. 

 He illustrates this conclusion by an experiment on a riband of iron, 

 magnetized nearly at an angle of 45 to its length, and heated along 

 one edge while the other is kept cool. "When the two ends, kept 

 at the same temperature, are put in communication with the elec- 

 trodes of a galvanometer, a powerful current is indicated, in such 

 a direction, that if pursued along a rectangular zigzag from edge to 

 edge through the band, the course is always from across to along the 

 lines of magnetization through the hot edge, and from along to across 

 the lines of magnetisation through the cold edge. 



4. In this part of the communication, attempts made by the author 

 to find the effects of various influences on electric conductivities of 

 metals are described. One of these, with a very unsatisfactory 

 method for testing resistances, led to the conclusion that longitudinal 

 magnetization diminishes the conducting quality of iron wire. The 

 general plan for testing resistances, which he subsequently adopted 

 as the best he could find, and which has proved very satisfactory, is 

 next explained ; and as an illustration, a single experiment on the 

 relative effect of an equal longitudinal extension on the resistances of 

 iron and copper wires is described. The conclusion established by 

 this experiment is, that both by extension with the tractive force 

 still in operation, and by permanent extension retained after a cessa- 

 tion of stress, the conductivity of the substance is more diminished 



