38 DR. F. HORTON ON THE EFFECTS OF CHANGES OF TEMPERATURE 



Observations on the effect of the amplitude of vibration on the period and 

 logarithmic decrement were made, as usual, at two temperatures, 15'00 C. and 

 99-00 C. 



It was found that as with most of the other wires examined both the period 

 and logarithmic decrement increased with the amplitude of vibration, and that the 

 rate of increase of each was greater at the higher temperature than at the lower one. 



Oscillating the vibrator through a large amplitude was found to increase both the 

 period and the logarithmic decrement at the smaller amplitude, indicating a 

 diminution in the rigidity and an increase in the internal viscosity of the wire. 



Aluminium. 



The wire used was of pure aluminium, -and the rigidity-temperature curve obtained 

 was similar to that given by silver. Several peculiarities were, however, noticed 

 with this wire. In the first place, heating to 59 C. diminished the value of the 

 rigidity modulus at 17 C., and this diminution was much larger after the wire had 

 been heated to 75 C., but after heating to 100 C. the rigidity at the laboratory 

 temperature was increased, and this increase was continued after each heating to 

 100 C. Successive heatings to 100 C. sometimes increased and sometimes 

 diminished the value of the rigidity modulus at that temperature. It is also 

 curious that, when kept at a constant temperature of 59 C. or 74 C., the rigidity 

 diminished slightly as time went on, but that at 100 C. it gradually increased with 

 time. It thus appears that the effect of heat on the rigidity of an aluminium wire 

 is very irregular. 



From the curve obtained, the modulus of rigidity for aluminium at 15 C. was 

 found to be 15 = 2'5478 X 10 11 dynes per sq. centim., and the value of the 

 "temperature coefficient" ft ='001351. 



The logarithmic decrements of the amplitudes of the torsional vibrations were 

 found to increase with the temperature, but the value at any one temperature 

 decreased as time went on. On plotting the logarithmic decrements against the 

 corresponding temperatures, the curve drawn through the mean of the observations 

 at each temperature was similar to that obtained for copper. 



Observations made at larger amplitudes of vibration showed that both the period 

 and logarithmic decrement increased with the amplitude, the increase being more 

 rapid at 98 '00 C. than at 19 '50 C. The effect of swinging through a large 

 amplitude (5) was to lessen the value of the logarithmic decrement, and to increase 

 the period of vibration at smaller amplitudes. 



Tin. 



A pure tin wire was used. Owing to the great internal viscosity of this metal, 

 the torsional periods could not be timed with the accuracy obtained in the case of 



