42 Mr. H. Tomlinson. The Influence of Stress [Apr. 28, 



interval between tlie period of permanent extension and that of 

 applying the load becomes greater. 



(2.) This increase of elasticity is greater in proportion for large 

 loads than for small ones. 



(3.) The increase of elasticity takes place whether the wire be 

 allowed to remain loaded or unloaded between the period of permanent 

 extension and that of the testing for the elasticity. 



(4.) The rate of increase of elasticity varies considerably with different 

 metals ; with some the maximum elasticity is apparently attained in a 

 few minutes, and with others not till some days have elapsed, iron and 

 steel being in this last respect very remarkable. 



(5.) The elasticity can also be increased by heavily loading and un- 

 loading several times, the rate of increase diminishing with each 

 loading and unloading. 



(6.) A departure from " Hooke's law " more or less decided, always 

 attends recent permanent extension even when the weights employed 

 to test the elasticity do not exceed one-tenth of the breaking weight. 



(7.) This departure is diminished very noticeably in the case of iron, 

 and much less so in the case of other metals, by allowing the wire to 

 rest for some time either loaded or unloaded ; it is also diminished by 

 repeated loading and unloading. 



With aluminium and zinc both the maximum temporary increase of 

 length caused by putting on weight, and the recovery on the removal 

 of the load are attained only after several hours, if the weights 

 employed be not very small. 



With tin and lead the loads employed were so small, in order to 

 avoid permanent set, that the values of " Young's modulus " obtained 

 for these metals cannot be relied on within 2 per cent. 



A small though decided departure from " Hooke's law " was found 

 in all cases, and the experiments abundantly proved that this law can 

 only hold good practically for much smaller loads than are usually 

 employed in determining the value of " Young's modulus " from the 

 method of static extension. 



A discussion of Wertheim's* experiments on elasticity will be found 

 in the paper, and it is there pointed out that a principal cause of the 

 differences of the values of " Young's modulus," obtained by him from 

 the method of static extension, and from longitudinal or transverse 

 vibrations, is to be found in his manner of experimenting according 

 to the first of these methods. 



The effect of permanent extension on the value of "Young's 

 modulus," was tried according to the direct method for iron and 

 copper, and indirectly for most of the metals. 



From both the direct and indirect methods results were obtained 

 which showed : — 



* " Ann. de Chem. et Phys." torn, xii, 1844. 



