of Vertically- suspended Wires, 



501 



Copper Wire. 



Number of 

 experiment. 



Scale-readings. 



Difference. 



Current direct. 



Current reversed. 



1. 

 2. 



3. 



569 

 567 

 595 



633 

 637 

 651 



-64 

 -70 



-56 



! 



Mean difference . 



.... -633 



Iron Wire. 



1. 

 2. 

 3. 



780 

 760 

 759 



770 



748 

 748 



+10 

 + 12 

 + 11 





Mean difference . 



+11 



I believe these effects are associated with certain thermo- 

 electric phenomena discovered by Sir William Thomson. In 

 his famous Bakerian lecture, published in the Philosophical 

 Transactions for 1856, he showed that if a stretched copper 

 wire is connected with an unstretched wire of the same metal 

 and the junction heated, a thermoelectric current will flow 

 from the stretched to the unstretched wire through the hot 

 junction ; while, if the wires are of iron, the direction of the 

 current will be from unstretched to stretched. It follows, 

 therefore, from the laws of the Peltier effect, that if a battery- 

 current is caused to flow from a stretched to an unstretched 

 wire, heat will be absorbed at the junction when the metal is 

 copper, and will be developed at the junction when the metal 

 is iron : and if the direction of the current is reversed the 

 thermal effects will also be reversed. 



Now a vertically suspended wire is unequally stretched by 

 its own weight, the stress gradually increasing from zero at 

 the lowest point to a maximum at the highest. Any small 

 element of the wire is more stretched than a similar element 

 immediately below it, and less stretched than a neighbouring 



