the Recalescence of Iron. 



105 



strength of the current. The internal friction of the iron was 

 measured by the logarithmic decrement of arc of the wire 

 when set in torsional oscillation. Part of the whole observed 

 decrement is due to the friction of the air, and part to the 



Fis 1. 



friction of the wire D against the mercury in H. The former 

 of these can be calculated from formulge given in the author's 

 paper on the internal friction of metals* ; the latter by ob- 

 serving the logarithmic decrement at the temperature of the 

 room, first when D dips into the mercury, and again when it 

 does not, whilst from this again the effect of the friction of 

 the mercury at higher temperatures can be approximately 

 determined provided the change wrought in the torsional 

 elasticity be known. The logarithmic decrement resulting 

 from the friction of the air and the friction of the mercury 

 conjointly was small compared with the whole observed 

 decrement, and was in all the calculations allowed for. 

 * Phil. Trans, vol. clxxvii. (part ii. 1886) pp. 813-815. 



