516 



Mr. H. Tomlinson. The Influence of 



The Effect of Longitudinal Traction on the Electrical Resistance of 



Magnesium. 



Cobalt and nickel nave been found to decrease in resistance when 

 subjected to moderate longitudinal traction, nor does it appear* that 

 this abnormal behaviour is in any way connected with the magnetic 

 properties of these metals. Nevertheless, it seemed desirable to find, 

 if possible, some metal which, possessing much feebler magnetic pro- 

 perties, would exhibit similar conduct. Now, judging from the 

 apparent relationship between the alteration of specific resistance 

 produced by traction and the " Hall effect," that magnesium might 

 be found to be such a metal, I obtained from Messrs. Matthey and 

 Johnson about 60 feet of magnesium wire, and examined it with the 

 above-mentioned object in view. 



Preliminary Determinations of the Value of " Young' 's Modulus" 

 Simple Rigidity, Density, 8fc. 



" Young's modulus " was, in the first instance, determined by the 

 method of longitudinal vibrations, a length of 550 cm. being under 

 •examination. The wire was stretched on a long wooden box, and 

 precautions taken which will be fully described in a future com- 

 munication to the Society, to avoid certain sources of error incidental 

 to this method. The results obtained with the syren were quite as 

 accordant with each other as those got with the cobalt strip, and 

 from these and the value of the density obtained in the manner pre- 

 sently to be described, was deduced a value of " Young's modulus " of 

 437*3 X 10 6 grams per square centimetre. 



The simple rigidity of the wire was determined by the method of 

 torsional vibrations in the manner described in a former portion of 

 this memoir,f and proved to be 172*3 X 10 6 grams per square centi- 

 metre. 



Some little difficulty was experienced in finding the density of the 

 magnesium, as when the wire was immersed in water a very large 

 number of minute bubbles of gas — evidently a consequence of chemi- 

 cal action — made the apparent mass in water from 10 to 20 per cent, 

 less than it ought to have been. Most of these bubbles could be 

 shaken off by merely moving the wire twice or thrice backwards and 

 forwards through the water, but in a very few seconds they collected 

 again and rendered it evident that no reliable determination of the 

 density could be obtained in this way. As there was not at hand any 

 liquid which might be suitably used in place of the water, the density 

 was ascertained from the length, diameter, and mass of three separate 

 lengths of the wire. The diameter of each of these pieces was mea- 



* '< Phil. Trans.," vol. 174, 1883, pp. 61, 62. 

 t Loc. cit., p. 24. 



