298 Messrs. J. H. Gray and J. B. Henderson. 



A comparison was also made of the ratio of the weight specific 

 resistance while the weight was on with that immediately after the 

 weight was taken off. The results are given in column 4 of the 

 table. Column 5, which is obtained by dividing the results of 

 column 3 by the corresponding numbers in column 4, gives therefore 

 the permanent alteration due to stretching. In this table both the 

 weights and the percentage stretchings are given, as it was found 

 that with iron the difficulty experienced with copper of continuous 

 stretching did not occur. 



In order to test if there was any more permanent stretching after 

 the wire had been allowed to rest for a week, another series of tests was 

 made on the same wire at the end of the week, readings being taken 

 on the application of every 7 Ibs. as before. The results showed that 

 the temporary alteration remained the same as before, being for 

 49 Ibs. 1 '002234, which, as will be seen by subtracting column 4 from 

 column 2 to get the temporary alteration, the result being for 49 Ibs. 

 1*00232, shows that the permanent alteration is practically constant 

 after the first series of stretchings. 



No tests have been made of alloys as yet, as it was thought more 

 important to give as much time as possible to obtaining trustworthy 

 results for pure metals. 



The conclusions that have been arrived at from this investigation 

 are that no mechanical treatment, such as stretching, drawing 

 through holes in a steel plate, twisting, hammering, or combinations 

 of these, all of which were tried, had any appreciable effect on the 

 electrical properties of copper, iron, or steel. 



The effect of annealing was also tried on a copper wire which had 

 already been very much stretched. The wire was carefully heated to 

 redness by means of a lamp all along its length, and a test then made. 

 This was not found to bring its resistance back to its original value 

 however. 



As contrasted with the small effect that mechanical treatment has 

 on the electrical properties of metals, it is interesting to notice the 

 great influence even a trace of impurity in the metal has. For this 

 purpose we include a table taken from Lord Kelvin's paper on 

 " Analytical and Synthetical Attempts to ascertain the cause of the 

 Differences of Electrical Conductivity discovered in Wires of nearly 

 Pure Copper " (vol. 2, ' Math, and Phys. Papers '). This table (p. 299) 

 gives an analysis, made by Professor Hofmann for Lord Kelvin, of 

 several specimens of copper, and also their conducting powers. 



From this table we see that an impurity of per cent, lowers the 

 conducting power by as much as 5J per cent., and that the conduct- 

 ing power rapidly becomes enormously lower for increase in the im- 

 purities, 1'24 per cent, of the latter bringing down by 40 per cent, of 

 its value when pure. 



