388 Mr. H. Tomlinson. The Influence of [Nov. 20, 



ance due to traction increase in greater proportion than the load. The 

 increase of length, however, increases less rapidly than the increase 

 of resistance, so that when the stress exceeds a certain limit, the above- 

 mentioned decrease of specific resistance is changed to increase. 

 Probably a similar state of things would be found to occur with alum- 

 inium. 



The value of "Young's modulus" was determined for magnesium, 

 both by the method of static extension and that of longitudinal vibra- 

 tions, the results obtained by the two methods agreeing fairly with 

 each other. The " simple rigidity " was determined by the method of 

 torsional vibrations. 



The value of the " bulk-modulus " was determined from the values 

 of " Young's modulus," and the " simple rigidity " to be 287'6 p X 10 

 grams per square centimetre, and this value agreed with that de- 

 duced from a formula given in a former portion of this memoir* 

 within 2 per cent. This formula is 



e r =2071x!0 6 xc,i, 



where e, is the " bulk- modulus " in grams per square centimetre, and 

 c, the mean -thermal capacity per unit volume between C. and 

 100 C. 



The results obtained from the experiments on the effects of stress 

 on the electrical specific resistance of magnesium are in perfect accord- 

 ance with the view that there is a relation between the " Hall effect " 

 and the alteration of specific resistance. 



The electrical resistance of the alloy platinum-iridiumf is murn more 

 increased by temporary longitudinal traction than that of either of 

 the components of the alloy. On the contrary the electrical resistances 

 of platinum-silver, German-silver, and brass are all considerably less 

 increased than is the case with their components. This circumstance 

 would rather militate against the employment of platinum- iridium in 

 making standard resistance coils. The increase of resistance alluded 

 to above is much greater than can be accounted for by change of 

 dimensions, so that the increase of specific resistance produced by 

 longitudinal traction is considerably greater than is the case with any 

 of those other metals examined whose resistance is increased by trac- 

 tion. 



'The alteration of resistance above mentioned at first increases in 

 greater proportion than the load, but, when a certain limit of stress 

 has been reached, the ratio of the temporary increase of resistance to 

 the load producing it begins to diminish, and finally reaches the same 



Received by the Royal Society, May 28, 1884, and as yet only read in 

 " Abstract." 



t 90 parts by mass of platinum and 10 parts of indium. 



