I'-M GENK11AL ( ONSIDKKATIONS. 



t\vo and a half tons in the larger ones, the variation of the 

 maximum and minimum values obtained by any om- 

 observer are rather large, and in many cases exceed 5 

 per cent. This large error is not so bad as it would seem 

 when it is remembered that the extensions \\ere those 



corresponding to very small increments of load, and also 

 i p i i 



that many <>t these maximum and minimum errors were 



probably accidental ones. Each observer calculated the 

 modulus of elasticity of the bars from the results of his 

 experiments. If the mean value of this modulus for all 

 loadings by all observers for each pair of similar bars be 

 taken as the correct value, it was found that the deviation 

 of the results of any one observer from this mean seldom 

 exceeded 1 per cent. In one case it was as low as one 

 hundredth of 1 per cent, and its mean value was some- 

 where about one-half of 1 per cent. Of course the different 

 values of the modulus as obtained by the several observers 

 do not vary wholly on account of the differences in the 

 instruments used, but were also affected by errors in the 

 measurements of the bars, errors of the testing machine, 

 and personal errors of observation. There was, unfortu- 

 nately, no attempt made to separate these errors from the 

 purely instrumental ones. The results of the large number 

 of tests thus carefully made do not .appear to show that 

 any one type of extensometer has a marked superiority 

 over the others. Some of the results nearest to the mean 

 were obtained by instruments of the simplest form. . 



