Effects of Mechanical Stress on Electrical Resistance. 295 



tion of specific resistance due to the strain caused by the weight, and 

 the permanent alteration due to the weight having been applied for 

 about ten minutes, and then taken off. This series was continued by 

 making successive additions of 14 Ibs. till the wire broke, which it 

 did when a little more than 200 Ibs. had been added. The results 

 obtained for one of four series are given in the table, p. 293. This 

 series was chosen because more intermediate readings had been taken 

 than in the other three. All four agreed at the common points within 

 the limits of accuracy of the method. 



The headings of the columns sufficiently explain what the numbers 

 mean. Column 4 shows that application of weight up to 84 Ibs. had 

 the effect of slightly improving the permanent electrical quality of 

 the steel. When more than 84 Ibs. were applied the conductivity 

 was found to diminish again. The results in column 2 are shown 

 plotted on the curves in Diagram 3. Columns 2 and 3 are almost 

 identical, as shown by column 4, which is the ratio of the two. The 

 numbers in column 4 are, within the accuracy of the method, practi- 

 cally unity, showing that there has been no permanent alteration of 

 specific resistance. With regard to the points a, a in each curve, 

 which are out to the amount of 5 in 10,000, it must be remarked 

 that these were the beginning of the second day's experiments. It 

 was impossible to carry out a whole series of tests in one day, and, 

 therefore, the wire was left over night with a weight of 14 Ibs. on it. 

 The temporary alteration of specific resistance in the case of steel 

 is considerably higher than the temporary or permanent alteration 

 for copper or iron, as will be seen from the results given for these 

 metals. 



Tests of Copper Wire. 



A very large number of tests were made on the effect of stretch- 

 ing on the specific resistance of copper wire. The wire was the 

 same material as that used for tests of density, but of slightly 

 smaller diameter. The following table gives the results of one set : 



(1.) (2.) 



Ratio of weight specific resistance 

 after weight was taken off to 

 that before any weight was 

 Percentage stretching. applied. 



0-0000 1-00000 



0-05 0-99969 



0-5 0-99985 



2-5 1-00104 



6-5 1-00415 



12-5 1-00666 



16-2 1-00682 



22-3 1-01083 (abnormal) 



VOL. MV. X 



