90 



Mr. H. Tomlinson. The Influence of 



The Longitudinal Elasticity as determined ly the Method of Statical 



Extension. 



The mode of experimenting by statical extension Las already been 

 described,* and the precautions which were used then were used now ; 

 but in these fresh trials the author availed himself of a device 

 whereby the departure from " Hooke's law," which had been formerly 

 observed more or less with all the wires, can be done away with. This 

 device the author owes to a perusal of the investigations of Professor 

 Gr. Wiedemann on statical torsion. f Wiedemann has proved that 

 though on first applying the loads used for twisting the wire the 

 torsional strain increases in greater proportion than the stress, the 

 frequent repetition of these loads gradually diminishes this want of 

 proportionality. He has farther shown that the process maybe much 

 facilitated by repeatedly putting the wire into torsional oscillations 

 whilst under the influence of the torsional stress. In a similar 

 manner the author now found that if the wire when under the 

 influence of a load causing longitudinal stress were set oscillating 

 longitudinally, by alternately pressing with the hand on the scale-pan 

 and removing the pressure, the range for which " Hooke's law " held 

 good was sensibly increased, X so that he was able to use larger loads 

 than could otherwise have been used without passing beyond the 

 boundaries of perfect elasticity. The following experiment will serve 

 to show the degree of accuracy attainable. 



* 'Phil. Trans.,' vol. 174, 1883 (Fart I), pp. 3, 4 

 + ' Phil. Mag.,' Jan. and Feb., 1880. 



X It should be mentioned here that in all cases it is advisable to allow a rest after 

 oscillating the "wire in this way for the first time, and afterwards to oscillate it again 

 j ist before testing. 



