

57 



cal theory to the phenomena of thermo-electricity in crystalline 

 metals, I have been led to experimental investigation on this branch 

 of the subject. The difficulty of obtaining actual metallic crystals 

 of considerable dimensions made it desirable to imitate crystalline 

 structure in various ways. The analogies of the crystalline optical 

 properties which have been observed in transparent solids, in a state 

 of strain, and of the crystalline structure as regards magnetic induc- 

 tion which Dr. Tyndall's remarkable experiments show to be pro- 

 duced not only in bismuth but in wax, thick paste of flour, and " the 

 pith of fresh rolls," by pressure, made it almost certain that pres- 

 sure or tension on a mass of metal would give it the thermo-electric 

 properties of a crystal. The only case which I have as yet had time 

 to try, verifies this anticipation. I have found that copper wire 

 stretched by a weight bears to similar copper wire unstretched, ex- 

 actly the thermo-electric relation which Svanberg discovered in a 

 bar cut equatorially from a crystal of bismuth or antimony compared 

 with a bar cut axially from a crystal of the same metal. Thus I 

 found that : 



If part of a circuit of copper wire be stretched by a considerable 

 force and the remainder left in its natural condition, or stretched by 

 a less force, and if either extremity of the stretched part be heated, 

 a current sets from t/ie stretched to the unstretched part through the 

 hot junction : and if the wire be stretched and unstretched on the 

 two sides of the heated part alternately, the current is reversed (as 

 far as I have been able yet to test, instantaneously) with each change 



the tension. 



I intend to make similar experiments on other metallic wires ; also 

 to try the effect of transverse as well as of longitudinal tension on 

 slips of sheet metal with their ends at different temperatures, when 

 placed longitudinally in an electric circuit ; and the effects of oblique 

 tension on slips of metal similarly placed in a circuit, but kept with 

 their ends at the same temperature and their lateral edges unequally 

 heated. I have no doubt of being able so to verify every thermo- 

 electric characteristic of crystalline structure, in metals in a state of 

 strain. 



Glasgow College, March 30, 1854 , 



P.S. April 19, 1854. I have today found by experiment that iron 



VOL. VII. G 



