258 Mr. S. Bidwell's Experiments illustrating 



Suppose now 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 unstrained, are at the same 

 potential. Then the ratio of the resistances between the points 

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

 the points 0, B and the points B, D. And so far as me- 

 chanical strain alone is concerned, this equality will not be 

 disturbed by placing the plate in a magnetic field, the strain 

 produced being 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 extension, and 

 (presumably) diminished by compression. But 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 come into play. 



It has already been mentioned that a thermoelectric cur- 

 rent will pass from a stretched copper wire to an unstretched 

 copper wire in contact with it, if the junction is heated. 

 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 

 I have proved by actual experiment that this is the case. 

 A fortiori, therefore, a current would pass through the heated 

 junction from a stretched copper wire to a compressed copper 

 wire. For similar reasons the current would, if the wires were 

 of iron, flow in the opposite direction, from the compressed 

 wire to the stretched wire , across the hot junction. And the 

 - same effects, so far as regards currents between stretched 

 and unstretched portions, occur, as I have proved experi- 

 mentally, if strips of foil are used instead of wires. If, there- 

 . fore, a battery-current is passed from a stretched portion of a 

 wire or foil to a compressed portion, heat will (according to 

 the laws of the Peltier effect) be absorbed at the junction if 

 the metal is copper, and will be developed at the junction if 

 the metal is iron. In passing from compressed to stretched 

 regions, the converse effects will occur. 



Let us now imagine the metal plate to be divided into four 

 equal regions A, B, 0, D, as shown in fig. 4. Let a current 

 pass through the plate from E to F, and let a force (produced 

 electromagnetically or otherwise) be applied in the direction 

 H G. First, suppose that the plate is of copper, then the 

 current travelling from E to the line G passes from a com- 

 pressed to a stretched portion of the metal ; heat will there- 

 fore be developed in the region A. Between the line G 



