Temperature Coefficient of Young's Modulus, 375 



resistance of the iron between the potential leads for a tem- 

 perature change of 2° was about 0*001 ohm, which could be 

 measured with certainty. Hence, combining this measure 

 of accuracy with that of the temperature coefficient, it is 

 estimated that temperature determinations in the main expe- 

 riment are correct to at least 3° C. over the whole range. 



6. Finally, combining the errors in the measurement of 

 the modulus, separate determinations of the latter are correct 

 to about 1*6 per cent., provided the temperature is constant 

 to within 0°-3 0. 



The Load Elongation Graphs. 



The method employed by most observers up to the present 

 time in finding the stretch due to a particular load, has been 

 to apply the latter several times in succession to the specimen 

 and take the mean of the resulting elongations. It is clear 

 that this process gives no direct information as to whether 

 Hooke's law holds or not ; and in none of those published 

 researches in which claim is laid to the existence of a maximum 

 in the elasticity, is there any direct experimental evidence of 

 the character of the load elongation curves. 



Harrison, in the investigation referred to on page 373, 

 found the stretches corresponding to a series of increasing 

 and decreasing loads, and was thus enabled to plot load elon- 

 gation graphs at each temperature, The same method was 

 used in the present experiments, the results of which are 

 described below. 



It is obvious that the forms of the load elongation curves 

 afford a definite test as to whether or not Young's modulus 

 reaches a maximum at a particular temperature for small load> 

 only. If a maximum exists (say at 0°, curve A, fig. 1) for loads 



Temperature 



below a certain critical value, and does not exist (curve B) 

 for loads greater than this value, it follows that the load 

 elongation graphs cannot be straight lines throughout the 

 range of stress employed. In other words, Hooke's law does 



