108 Mr. Herbert Tomlinson on 



to demonstrate that all musical sound departs from the bell 

 when struck provided the temperature approach a visible red. 

 This absence of musical sound is generally, if not always, 

 attributed to diminution of elasticity; but it would seem, from 

 the above, to be much rather attributable to increase of 

 internal friction*. 



Another point to be noted in this experiment is the large 

 permanent increase of internal friction produced by the rise 

 of temperature. This doubtless arose from the fact that, 

 though the mass of the suspended system only amounted to 

 40 grms., there was a sensible permanent lengthening of the 

 wire resulting from the rise of temperature ; for the author 

 has shown that permanent extension and permanent torsion 

 may both increase very considerably the vnluo of the loga- 

 rithmic decrement. 



An equally important point to the above is the effect of rest 

 on the logarithmic decrement when the wire has recently had 

 its temperature changed. It seems not improbable that, large 

 as the internal friction is at the higher temperatures, it would 

 be larger if tested immediately after the change had taken 

 place. Again, it would be larger still if tested by a statical 

 method ; for the preliminary vibrations, to which the wire 

 was subjected before testing, evidently reduced considerably 

 the internal friction. 



The Two Critical Temperatures of Iron. 



The above experiment seems to show, not only that the 

 internal friction of iron is very considerable at high tempera- 

 tures, but also that there are two points, namely about 550° C. 

 and 1000° C, at which there are very sudden changes in the 

 rate at which the friction increases with the temperature. 

 Now at or near these points there are, at any rate for annealed 

 iron wire of good quality, very notable and sudden changes in 

 certain of the physical properties of the metal. About 550° C. 

 the metal begins to lose its magnetic properties very rapidly; 

 so rapidly indeed that, if an iron wire be surrounded by a 

 magnetizing solenoid always kept in action, and this again be 

 concentric with a secondary solenoid connected with a galva- 

 nometer, a very sensible induced current can be observed at 

 the critical temperature both on heating and cooling. It would 

 seem likely also, from the researches of Prof. P. G. Tait and 

 others, that at this temperature, or about this temperature, 

 there are sudden changes in the thermoelectrical properties of 



* The elasticity of iron at 1000° C. is nearly the same as the elasticity 

 of silver at 20° C. 



