92 



Mr. H. Tomlinson. The Influence of 



The mean value of AS/AP for the four series is 1-1969 and the 

 probable error is O10 per cent. The values of AS/AP obtained with 

 the temporary stresses of 2 kilos., 4 kilos., 6 kilos., and 8 kilos, are, 

 within the limits of errors of observation, equal, and moreover are 

 independent of the permanent load on the wire. 



The results recorded above were not got without the exercise of 

 very great precaution both in adjusting the scale and vernier, and in 

 preventing either the wire under examination or the comparison-wire 

 from twisting after the adjustment had been completed. Should such 

 twisting take place the vernier will not move with sufficient freedom 

 up and down the scale.* Indeed in this experiment there are here 

 and there slight traces of the vernier sticking in the scale, and in 

 most of the other experiments it was considered advisable to unclamp 

 both scale and vernier and completely readjust them, so that slightly 

 different lengths of the wire might be under examination, f The next 

 experiment furnishes an example of this mode of treatment. 



Experiment III. — Piano-steel. 

 Temporary load, 8 kilos. ; permanent load, 10 kilos. 



Length of the wire 

 under examination 

 in centimetres, /. 



Temporary alteration 

 of length in centi- 

 metres, AS. 



AS/2. 



753-8 



0-5245 



-0006955 



750-8 



'5195 



-0006920 



754-0 



0-5233 



-0006940 



The mean of the numbers in the third column is 0*0006938, with a 

 probable error of 0'09 per cent. It is evident therefore that with 

 care considerable accuracy can be obtained. The elasticity of piano- 

 steel within the limits of the loading here employed had been pre- 

 viously proved to be quite perfect, provided the precautions adopted 

 in this and the other experiments were taken, so that it was considered 

 unnecessary to try the effect of lesser loads, but with all the other 

 metals at least two loads were used, and for these loads Hooke's law, 

 "ut extensio sic vis," could by proper treatment of the wire be made 

 to hold good within the limits of errors of observation. 



* The author may mention here that he found it to he a great convenience to 

 place four blocks of well-planed wood on the ground close to the four sides of the 

 scale-pan, and to anchor the bar fastened to the comparison- wire by fine wires 

 attached to the ends of the former and secured in a horizontal position, 



f The different lengths were not measured until the series of temporary altera- 

 tions of length produced by the various changes of load was com plated, so as to 

 avoid prejudice on the part of the observer. 



