344 



Mr. S. Bidwell. On an 



[Feb. 21, 



Suppose that a current is passing through the plate from C to D, 

 and that A and B are two points on the opposite edges which, when 

 the metal is free from strain^ are at the same potential. Then the 



Fig. 3. 



Compressed portions are shaded. Stretched portions are blank. 



ratio of the resistances between the points C and A and the points 

 A and D is equal to that between the points C, B, and the points 

 B, D. And, so far as mechanical strain alone is concerned, this 

 equality will not be disturbed by placing the plate in a magnetic 

 field, for the strain produced will be symmetrically distributed on 

 both sides of the middle line. At all events, no strain could occur 

 which would in itself affect the resistance of gold and iron in opposite 

 ways, for the resistance of both is increased by tension and (pre- 

 sumably) diminished by longitudinal compression. But it will be 

 noticed that the currents from C to A and from B to D pass from 

 regions which are compressed to regions which are stretched, while 

 the opposite is the case with currents passing from C to B and from 

 A to D. And here the thermoelectric effects above referred to come 

 into play. 



Sir William Thomson, in his Bakerian Lecture of 1856 (" Phil. 

 Trans.," 1856, p. 711) announced his discovery of the fact that if a 

 stretched copper wire is connected with an unstretched wire of the 

 same material, 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. From this it might be 

 inferred that a current would flow through the heated junction from 

 an unstretched, or free, copper wire to a longitudinally compressed 

 copper wire, and from a longitudinally compressed iron wire to a free 

 iron wire ; and I have ascertained by actual experiment that this is 

 $ the case, though with copper the observed effect was very small. A 

 fortiori, therefore, the direction of the current through the heated 

 junction will be from stretched to compressed in the case of copper 

 wire, and from compressed to stretched in the case of iron wire. If, 



