1881.] and Strain on the Action of Physical Forces. 45 



which e = " Young's modulus," r =rigidity, both in grams per square 

 centimetre, and <r = the required ratio. 



In the following table are given the values of e, r, and a for most of 

 the annealed metals : — 



Name of metal. 



" Young's 

 modulus." 



e. 



Torsional 

 rigidity. 



r. 



Eatio of lateral 

 contraction to 

 elongation. 





1981 x 10 6 



773 -1 x 10 6 



•281 





1490 • 



692-7 



•076 





1335 



493 -7 



•354 





1160 



440-6 



•315 





1051 



369-9 



•420 





767 



338-4 



•133 





742 



271-8 



•367 



Aluminium 



673 



265-2 



•269 



Tin 



277 



120-9 



145 





167 



74 -0 



•136 



The mean value of a for the different substances employed in the 

 annealed condition ="2515, a number closely according with that 

 assigned by Poisson from mathematical considerations, as the value of 

 a for each. 



The metals copper, platinum, aluminium, silver, and platinum- 

 silver were obtained from Messrs. Johnson, Matthey, and Co., as 

 chemically pure, and the zinc, lead, and tin wires as being as pure as 

 could be got from the ordinary process of distillation. 



Experiments on the permanent alteration of density which can be 

 produced by longitudinal traction proved, as the investigations of 

 Wertheim and Thomson have already shown, that such alteration is 

 very slight. In these particular experiments no change amounting to 

 |- per cent, was detected, though the wires were strained to breaking. 



Certain calculations were made to verify Wertheim's formula 

 ftXa 7 =a constant, where e=" Young's modulus," and a=the mean 

 distance between one molecule and another, and it was found that 

 this formula was approximately true. Moreover, the products of r 

 and a, 7 were calculated and found to be approximately constant for 

 the different metals. 



The influence of an electric current and of magnetism on the tor- 

 sional rigidity of metals was also investigated, and the following 

 results arrived at : — 



(1.) The torsional rigidity of copper and iron is temporarily decreased 

 by the passage of a powerful electric current, but is very little, if at all 

 appreciably, altered by currents of moderate intensity. 



(2.) The torsional rigidity of iron is temporarily diminished to a 

 small but perceptible extent by a high magnetising force. 



