553 



To test whether or not the mechanical quality of the metal as to 

 hardness or temper had any influence on the electrical conducting 

 power, the following comparison was made between a piece of soft 

 No. 18 wire, and another piece of the same pulled out and hardened 

 hy weights applied up to breaking. 



The result shows that the greatest degree of brittleness produced by 

 tension does not alter the conductivity of the metal by as much as 

 one half per cent. A similar experiment showed no more sensible 

 effect on the conductivity of copper wire to be produced by hammering 

 it flat. There are, no doubt, slight effects on the conductivity of 

 metals, produced by every application and by the altered condition 

 left after the withdrawal of excessive stress*; and I have already 

 made a partial examination of these effects in copper, iron, and pla- 

 tinum wires, and found them to be in all cases so minute, that the 

 present results as to copper wire are only what was to be expected. 



To find whether or not there is any sensible loss of conducting 

 power on the whole due to the spiral forms given to the individual 

 wires when spun into a strand, it would be well worth while to com- 

 pare very carefully the resistances of single wires with those of strands 

 spun from exactly the same stock. This I have not yet had an 

 opportunity of doing ; but the following results show that any defi- 

 ciency which the strand may present when accurately compared with 



* See the Bakerian Lecture, " On the Electro-dynamic Qualities of Metals," 

 104, 105 and 150, Philosophical Transactions for 1856. 



