ON THE MODULUS OF TORSIONAL RIGIDITY OF METAL WIRES. 35 



logarithmic decrement increases as the temperature is raised, slowly at first, but more 

 rapidly at higher temperatures. This was especially noticeable after passing 100C. 



Experiments were made to ascertain the manner in which the period and logarithmic 

 decrement varied with the amplitude of vibration of the vibrator. Observations were 

 taken at two temperatures, viz., 19'0C. and 99 '3 C. It was found that the 

 logarithmic decrement curves for the two temperatures were very much alike, and 

 showed that the logarithmic decrement increases with the amplitude of vibration, the 

 rate of increase being slightly greater at the higher temperature. This is practically 

 the same as the result obtained for platinum. 



From the curves for the variation of period with the amplitude of vibration it was 

 evident that the period increases with the amplitude, the rate of increase at the 

 higher temperature being greater than at the lower. 



Steel. 



Two wires of pianoforte steel were experimented on, one in the first form 01 

 apparatus in which the vibrator was not enclosed in the heating jacket, and the other 

 with the apparatus in its final form. Both were carefully annealed in the manner 

 already described before the determinations of the rigidity were begun. With the 

 first wire the method of experimenting was slightly different from that used with the 

 other wires. The temperature in the heating jacket was regulated by means ot 

 a stream of water heated by passing through a gas furnace fitted with a quiqkly 

 acting gas regulator. By this means the temperature could be kept constant for any 

 length of time, and the effect of very small alterations of temperature observed. 

 With this arrangement the temperature could not be raised above 36 C., but 

 observations at 75C. and 100C. were also taken by employing the method of 

 heating described earlier in this paper. I do not attach much importance to the 

 numbers obtained, on account of the defective form of heating jacket employed, but 

 the results can be relied on in so far as they show that the rigidity modulus of steel 

 is a perfectly definite quantity, and that its rate of decrease with increasing 

 temperature is constant also. 



Table VII. contains the values of the rigidity modulus at the various temperatures. 

 Each was calculated from the mean of a large number of observations (in most cases 

 more than 20) at the temperature given. It must be stated that the results have 

 1 .-I! tubulated in deso nding order of temperature not in the order in which they 

 were obtained. This accounts for the fact that in two or three cases results are given 

 at very nearly the same temperatures. These were taken at different times, weeks 

 apart, in order to see if the rigidity came tack to its original value after the wire had 

 been heated to a higher temperature. 



F 2 



