ON THE MOnm.US OF TORSIONAL RTOTDITY OF METAL WIRI - 41 



much greater at higher temperatures than ;it the ordinary temperature of the 

 laboratory. 



Some observations were made at llie temperature of the lalx>rat<>i v and at 1<>0 C O., 

 MI a lead wire \vliicli had lieen carefully annealed. It was found that the rigidity 

 increased only very slowly with the time, and that the increase per 1 C. over this 

 range of temperature was nearly the same as with the first wire. This might j)erhaps 

 have l)een expected, for in the final values of the modulus of the first wire a certain 

 amount of annealing must have taken place owing to the wire having l>een kept for 

 some time at 100 C. The increase of the modulus per 1 C. was for the annealed 

 wire '0130 X 10 10 dynes per sq. centim., and for the unannealed wire '0127 X 10 10 

 dynes per sq. centim. The value of the modulus at 15 C. tin the annealed wire was 

 n, 5 = 7'977 X 10' dynes per sq. centim. 



Observations of the logarithmic decrement of the amplitudes of the torsional 

 oscillations of this wire gave results similar to those obtained with the unannealed 

 wire. 



Cadmium. 



The wire was of pure cadmium, and was first annealed in the manner already 

 described. The small rectangular brass vibrator was used, but its weight caused the 

 wire to elongate very considerably at the higher temperatures, and the results 

 obtained at those temperatures were therefore unreliable. A new wire was then 

 used, and the temperature coefficient determined from ol>servation8 at the temperature 

 of the laboratory and at 32 C., the experiments l)eing done as quickly as possible in 

 order that the radius of the wire might l>e determined l>efore it had drawn out to any 

 serious extent. The total elongation was '052 centim., and a correction on this 

 account was applied in determining the rigidity. 



The value of the rigidity modulus at 15 C. was found to be 7 l& = 2'3124 X 10 n 

 dynes per sq. centim., and the "temperature coefficient" '005855. 



The logarithmic decrement of the amplitude of the torsional oscillations increased 

 with the temperature, the rate of increase being much greater at 100 C. than at the 

 ordinary laboratory temperature. 



Commercial Copjier. 



An ordinary copper wire of the same size as the other wires used was carefully 

 annealed in an atmosphere of carlxinic acid gas, and a short series of observations 

 taken with it, the object being to ascertain if there was any striking difference 

 between the behaviour of this wire and that of the pure copper wire already 

 descril>ed. 



It was found that the rigidity at a constant temperature was far from being 

 constant, in marked contrast to what was obtained with the pure copper wire. On 

 plotting a series of values of the modulus of rigidity against the corresponding 



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