1885.] 



Electric Resistance of Platinoid. 



343 



0*02087. A second wire tested very carefully in a similar way gave 

 for this average percentage variation between 0° and 100°, 0*022 per 

 degree, with a steadily increasing rate of variation from the 

 beginning. 



To compare this increase in resistance due to increase of tempera- 

 ture with that observed in other metals and alloys, I find that the 

 percentage increase of resistance for 1° C. at 20° C. for copper is 

 0*388, platinum- silver alloy 0*031, gold-silver alloy 0*065, and for 

 German silver 0*044. These numbers were obtained by Matthiessen 

 in the course of his experiments for finding a suitable metal or alloy 

 for the purpose of constructing the British Association standards of 

 electric resistance. It appears that the variation of resistance of 

 platinoid with temperature is very much smaller than the smallest 

 observed for any of the metals and alloys then examined. 



Addition to the above Paper. Received May 12, 1885. 



I have now (May 7th, 1885) obtained some information as to the 

 mechanical properties of platinoid wire. 



I have determined the modulus of rigidity of a wire of the substance, 

 a portion of the wire marked A in the preceding paper being used for 

 the purpose. This wire is a little larger than No. 24 wire of the Board 

 of Trade Standard wire gauge, and has a diameter of 0*0595 cm. To 

 determine the modulus of rigidity a cylindrical ring vibrator was 

 attached by a cross-bar to the lower end of a length of the wire, 

 the upper end being fixed by soldering to a brass plate,, which was 

 screwed to a beam in the roof of the laboratory. The moment of 

 inertia of the cylindrical vibrator and cross-bar was ascertained ; and 

 the torsional vibrations of the vibrator hung by the wire were counted, 

 and the period determined* From this, and the dimensions of the 

 wire, the modulus of rigidity is calculated. 



The following are the particulars of the experiment : — 



Length of wire used (V) 490*8 cm. 



Diameter of wire (2a) 0*0595 „ 



Moment of inertia of vibrator (MA: 2 ) 29453*2 



Time of vibration one way (half period) (T) 16*375 sees. 

 Rigidity in grammes weight per square cm. 



™) 475*8x10*. 



grTV ) 



The Young's Modulus, or modulus for elastic longitudinal exten- 

 sion, and the breaking weight, have been determined for me by 

 Mr. Magnus Maclean, Official Assistant to the Professor of Natural 



* Sir William Thomson, " Elasticity and Viscosity of Metals," " Proc. Roy. 

 Soc," 1865. 



VOL. XXXVIII. 2 B 



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