172 



Dr. M. Maclean. * On the Effects of 



for all the wires tried with weights on and off, except for soft copper 

 and iron. The greatest permanent elongation produced in any of the 

 hard copper wires experimented on was 0*17 per cent., and for this 

 permanent elongation the reading on the galvanometer was in the same 

 direction for weights off and on, though always greater for the latter. 



§ 14. For the soft copper wire (Table IX below) the readings were in 

 the same direction for weights on and off, up to a permanent elongation 

 of 1 per cent. After a permanent elongation of 4*72 per cent., the 

 current with weight on was O'OOIOS mikroampere per degree from 

 stretched to unstr etched through the hot junction, while with the weight 

 off, the current was 0*00075 mikroampere per degree from unstr etched 

 to stretched through the hot junction. 



For iron wire the current was in the same direction for weights on 

 and off, up to a permanent elongation of 0*35 per cent. ; but after a 

 permanent elongation of 3*41 per cent, the current with weight on was 

 '00461 mikroampere per degree from unstr etched to stretched through 

 the hot junction, and with weight off, 0*0069 mikroampere per degree 

 from stretched to unstr etched through the hot junction. 



§ 15. In 'Mathematical and Physical Papers,' vol. 2, p. 270, § 109, 

 Kelvin says : — " I have thus arrived at the remarkable conclusion that 

 when a permanent elongation is left after the withdrawal of a longi- 

 tudinal force which has been applied to an iron or copper wire, the 

 residual thermo-electric effect is the reverse of the thermo-electric effect 

 which is induced by the force, and which subsists as long as the force 

 acts." 



It seems (1) that for small longitudinal strain in copper or in iron 

 the direction of the current through the hot junction is the same, whether 

 the force which produced the permanent strain is on or off, (2) that 

 as the permanent elongation is increased by increased longitudinal 

 forces, a stage is reached which gives zero current when the forces are 

 removed, and (3) that for greater longitudinal forces and permanent 

 elongations the direction of the current is opposite, with the pulling 

 forces off and on. It seems, in fact, that the permanent elongation must 

 exceed a definite limit, to produce reverse thermo-electric effects with 

 the longitudinal force on and removed. I hope to further investigate 

 this point and to report the results to the Society. 



§ 16. The galvanometer used for the investigation of these temporary 

 and permanent strains was one of the Kelvin recorder pattern, namely, 

 a movable coil between the poles of a strong permanent magnet of 

 circular form. The coil had 81 turns and a resistance 14'94 ohms at 

 \T b C. Its constant was determined in the usual w^ay, and found to 

 be 0'029 mikroampere per division of the scale. 



§ 17. To find the stress-strain diagram, experiments were performed 

 on a specimen of rach wire, in the following manner. Two pieces of 

 the wire were passed through two small holes in a metal plate, and 



